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RICHARD  WESTBROOK  LECTURES  OF  THE  WAGNER  FREE  INSTITUTE 
OK  SCIENCE,  PHILADELPHIA. 


THE  THEORY  OF  EVOLUTION 


THE  MACMILLAN  COMPANY 

HEW  YORK  •   BOSTON  •   CHICAGO   •  DALLAS 
ATLANTA   •   SAN  FRANCISCO 

MACMILLAN  &  CO.,  LIMITED 

LONDON   •  BOMBAY   •  CALCUTTA 
MELBOURNE 

THE  MACMILLAN  CO.  OF  CANADA,  LTD. 

TORONTO 


THE 
THEORY  OF  EVOLUTION 


WITH  SPECIAL  REFERENCE  TO  THE  EVIDENCE 
UPON  WHICH  IT  IS  FOUNDED 


BY 
WILLIAM  BERRYMAN  SCOTT 

I.D.    (HEIDELBERG),  HON.   D.SC.    (HARVARD  &  OXFORD),   LL.D. 

(UNIVERSITY  OF  PENNSYLVANIA),   BLAIR  PROFESSOR  OF 

GEOLOGY  &  PALEONTOLOGY  IN  PRINCETON  UNIVERSITY 


Nnu  fork 
THE  MACMILLAN  COMPANY 

1921 

All  rights  reserved 

39510 


COPYBIGHT,  1917 

BY  THE  MACMILLAN  COMPANY 
Set  up  and  electrotyped.     Published  February,  1917. 


QM 

iim       36  ^ 

S43|-fc 
1917 

;>7*:  •  \.  co p.  2j 

PREFACE 

A  new  book  on  evolution,  which  can  lay  little 
claim  to  novelty  of  fact  or  treatment,  certainly 
demands  an  explanation,  if  not  an  apology.  My 
choice  of  subject  for  the  Westbrook  lectures  of  1914 
was  determined  by  the  very  general  misapprehension 
in  the  public  mind  concerning  the  present  status  of 
the  evolutionary  theory  among  men  of  science.  It 
is  widely  believed  that  the  theory  is  an  outworn 
device,  which  naturalists  are  beginning  to  discard 
and  that  soon  it  will  have  a  merely  historical  in- 
terest. This  misunderstanding,  for  such  it  is,  has 
arisen  from  the  debates  among  zoologists  and  botan- 
ists as  to  the  manner  in  which  evolution  has  actually 
occurred  and  the  efficient  causes  which  have  brought 
it  about,  and,  further,  from  the  ambiguous  way  in 
which  the  term  "Darwinism"  is  often  employed. 
Frequently,  the  term  is  made  a  synonym  of  evolu- 
tion, but  it  ought  properly  to  be  restricted  to  Dar- 
win's explanation  of  evolution  by  natural  selection. 

It  seemed  that  a  useful  service  might  be  rendered 
by  making  an  outline  review  of  the  evidence  upon 
which  the  doctrine  of  evolution  is  founded,  for  the 
nature  and  scope  of  this  evidence  are  but  little  under- 
stood by  the  educated,  though  non-scientific  pub- 
lic. The  interest  displayed  in  these  topics  by  the 


vi  PREFACE 

audiences  at  the  Wagner  Institute  encourages  me 
to  hope  that  a  larger  and  more  widely  dispersed 
audience  may  share  the  same  interest.  The  annoy- 
ing barrier  of  technical  language  has  been  evaded 
so  far  as  possible,  but  the  lack  of  suitable  vernacular 
terms  is  such  that  it  is  not  practicable  to  escape 
technicalities  altogether.  The  effort  to  use  ordinary 
speech  and  the  want  of  numerous  illustrations  have 
caused  a  certain  prolixity  of  description,  which  is 
regrettable,  but  less  so,  perhaps,  than  the  alternative 
of  obscurity. 

My  experience  with  graduate  students  of  biology 
has  shown  me  that  usually  their  training  has  so  com- 
pletely taken  for  granted  the  truth  of  the  evolu- 
tionary doctrine,  that  they  have  but  a  vague  con- 
ception of  the  testimony  by  which  that  doctrine 
is  supported.  This  book  may  be  useful  in  directing 
their  attention  to  the  character  of  the  evidence  and, 
though,  of  course,  the  meagre  sketch  contained  in 
these  lectures  is  entirely  inadequate  in  itself,  it  does 
provide  an  outline  which  students  can  easily  fill  in 
from  their  own  reading. 

In  the  winter  of  1910-11  an  admirable  series  of 
popular  lectures  by  several  men  of  high  distinction 
was  delivered  in  the  Natural  Science  Society  of 
Munich,  as  a  tribute  to  the  centennial  of  Charles 
Darwin's  birth.  These  lectures  were  published  in 
1911  by  G.  Fischer  of  Jena  with  the  title  Die  Abstam- 
mungslehre.  To  this  most  excellent  compendium 
of  the  evidences  of  evolution  I  am  under  great  ob- 


PREFACE  vii 

ligations  and  I  wish  that  it  might  soon  be  translated 
into  English.  I  take  pleasure  also  in  expressing  my 
thanks  to  my  colleagues,  Professors  Conklin  and 
Smyth  and  Dr.  Mayer  for  valuable  suggestions  in 
the  difficulties  of  treatment  and  to  Dr.  W.  J.  Sinclair 
for  his  care  in  making  the  figures. 

W.  B.  S. 

PRINCETON.  N.  J.,  December,  1916. 


CONTENTS 

LECTURE  I 

Present  Status  of  the  Question   .     . 


LECTURE  II 

Evidences  for  the  Theory — Classification,  Domestica- 
tion and  Comparative  Anatomy 27 

LECTURE  III 

Evidence  from  Embryology  and  Blood  Tests    ...    57 

LECTURE  IV 

Evidence  from  Palaeontology 82 

LECTURE  V 

Evidence  from  Geographical  Distribution     ....  120 

LECTURE  VI 
Evidence  from  Experiment — Conclusion 149 


THE  THEORY  OF  EVOLUTION 


THE  THEORY  OF  EVOLUTION 

LECTURE  I 
PRESENT    STATUS    OF    THE    QUESTION 

Among  that  public  which,  though  educated  and 
intelligent,  is  yet  not  professionally  scientific,  there 
has  been,  of  late,  a  widespread  belief  that  naturalists 
have  become  very  doubtful  as  to  the  truth  of  the 
theory  of  evolution  and  are  casting  about  for  some 
more  satisfactory  substitute,  which  shall  better  ex- 
plain the  infinitely  varied  and  manifold  character  of 
the  organic  world.  This  belief  is  an  altogether  mis- 
taken one,  for  never  before  have  the  students  of 
animals  and  plants  been  so  nearly  unanimous  in 
their  acceptance  of  the  theory  as  they  are  to-day. 
It  is  true  that  there  are  still  some  dissentient  voices, 
as  there  have  been  ever  since  the  publication  of 
Darwin's  "Origin  of  Species,"  but  the  whole  trend 
of  scientific  opinion  is  strongly  in  favour  of  the 
evolutionary  hypothesis. 

Whatever  may  be  the  private  opinion  of  a  greater 
or  less  number  of  naturalists  on  this  question,  almost 
the  only  zoologist  of  recognized  standing  who  has 
taken  a  pronounced  and  positive  position  against  the 
theory,  is  Professor  Fleischmann,  of  Erlangen.  He 


2  THE  THEORY  OF  EVOLUTION 

writes:  "The  more  deeply  I  pursued  the  alleged  evi- 
dence for  it  [the  theory]  and  sought  to  gain,  through 
special  investigations,  some  essential  proof  of  the 
genetic  relationships  of  animals,  the  more  clearly  I 
recognized  that  the  theory  is  a  seductive  romance, 
which  deceptively  pretends  to  give  results  and  ex- 
planations, rather  than  a  doctrine  built  upon  positive 
foundations."  1  In  another  passage  he  speaks  of  the 
"collapse  of  the  theory."  This  opinion  of  Professor 
Fleischmann's  stands  almost  entirely  alone  in  modern 
biological  literature,  yet  is  not  altogether  surprising, 
for,  from  the  very  nature  of  the  case,  such  complete 
and  indubitable  proof  as  Professor  Fleischmann  de- 
mands is  unattainable. 

What,  then,  has  led  to  the  belief,  mentioned  at 
the  outset,  that  naturalists  have  so  largely  grown 
sceptical  and  are  inclined  to  abandon  the  theory? 
This  belief  is  principally  due  to  the  fact  that  there 
is  no  agreement  among  men  of  science  as  to  the 
manner  in  which  evolution  operates  (modes)  or  as 
to  its  efficient  causes  (factors)  and,  further,  to  the 
often  fierce  controversies  which  long  have  been  and 
still  are  held  concerning  these  problems  of  modes 
and  factors.  Some  biologists  maintain  that  these 
questions  are  unanswerable  in  the  present  state  of 
knowledge  and  have  taken  up  other  lines  of  investi- 
gation, but  that  is  very  far  from  being  an  abandon- 
ment of  the  theory  of  evolution  itself.  It  is  one 
thing  to  accept  a  fact  as  substantially  proved  and 

1  Albert  Fleischmann:  Die  Descendenztheorie.  Leipzig,  1901.  p.  iii. 


PRESENT  STATUS  OF  THE  QUESTION        3 

quite  another  thing  to  devise  a  satisfactory  explana- 
tion of  that  fact. 

A  few  quotations  from  distinguished  investigators, 
who  are  representatives  of  different  schools  of 
thought,  will  serve  to  show  just  what  the  most  strik- 
ing divergences  of  opinion  are.  Professor  E.  B.  Wil- 
son, in  his  address  as  President  of  the  American  As- 
sociation for  the  Advancement  of  Science  (1914), 
says:  "Biologists  turned  aside  from  general  theories 
of  evolution  and  their  deductive  application  to  spe- 
cial problems  of  descent,  in  order  to  take  up  objective 
experiments  on  variation  and  heredity  for  their  own 
sake.  This  was  not  due  to  any  doubts  concerning  the 
reality  of  evolution  or  to  any  lack  of  interest  in  its 
problems.  It  was  a  policy  of  masterly  inactivity  de- 
liberately adopted;  for  further  discussions  concerning 
the  causes  of  evolution  had  clearly  become  futile  until 
a  more  adequate  and  critical  view  of  existing  genetic 
phenomena  had  been  attained"  [Italics  mine.] 

Professor  Gustav  Steinmann,  of  Freiburg,  who 
occupies  a  very  isolated  position  regarding  the  actual 
lines  of  descent  and  relationship  between  the  various 
groups  of  plants  and  animals,  and  whose  views  on 
these  topics  have  been  very  severely  criticised,  has 
no  hesitation  in  declaring  his  unreserved  acceptance 
of  the  evolutionary  theory.  His  words  are:  "The 
theory  of  development,  as  it  was  revived  by  Darwin 
nearly  half  a  century  ago,  is  in  its  modern  form  pre- 
vailingly unhistorical.  True,  it  has  forced  beneath 
its  sceptre  the  methods  of  investigation  of  all  the 


4  THE  THEORY  OF  EVOLUTION 

sciences  which  deal  with  the  living  world  and  to-day 
almost  completely  controls  scientific  thought.  .  .  . 
And  yet  science  does  not  sincerely  rejoice  in  its  con- 
quests. Only  a  few  incorrigible  and  uncritically  dis- 
posed optimists  steadfastly  proclaim  what  glorious 
progress  we  have  made;  otherwise,  in  scientific  as 
in  lay  circles,  there  prevails  a  widespread  feeling  of 
uncertainty  and  doubt.  Not  as  though  the  correctness 
of  the  principle  of  descent  were  seriously  questioned; 
rather  does  the  conviction  steadily  grow  that  it  is  in- 
dispensable for  the  comprehension  of  living  nature, 
indeed  self-evident.  [Italics  mine.]  But  never  before 
has  it  become  so  notorious  as  in  the  last  decade,  how 
little  there  is  in  this  theory  that  is  universally  ac- 
cepted, as  appears  when  the  most  obvious  questions 
are  asked  regarding  the  course  of  development  and 
its  efficient  causes.  No  one  can  supply  simple  and 
satisfactory  answers.  Should  any  one  ask,  by  what 
steps  the  placental  mammals  have  proceeded  from 
lower  quadrupeds,  he  is  informed  that  some  inves- 
tigators derive  them  from  the  reptiles  by  way  of 
the  marsupials,  others  directly  from  certain  Permian 
reptiles,  while  others  again  push  back  their  origin 
to  the  most  ancient  periods  and  derive  them  from 
entirely  unknown  ancestors,  or,  should  somebody 
wish  to  know  why  the  many  remarkable  animals 
and  plants  of  preceding  geological  periods  are  no 
longer  living,  and  what  causes  led  to  their  extinction, 
he  would  receive  anything  but  a  clear  and  unam- 
biguous answer.  Who  would  venture  to-day  to 


PRESENT  STATUS  OF  THE  QUESTION        5 

report  anything  but  the  personal  conception  of  in- 
dividual investigators,  if  he  were  asked  for  infor- 
mation regarding  the  hereditary  transmission  of 
acquired  characters,  the  significance  of  natural  selec- 
tion, or  many  other  details  of  the  theory  of  evolution? 
For  what  to  one  is  the  corner  stone  of  the  theory  is 
to  another  a  factor  of  quite  subordinate  importance, 
while  a  third  regards  it  as  the  greatest  aberration 
of  the  past  century."  1  Lastly,  may  be  cited  Mr. 
Bateson,  one  of  the  foremost  exponents  of  Mendelism, 
who  rejects  Darwinism,  one  might  say,  with  con- 
tumely. "  The  many  converging  lines  of  evidence  point 
so  clearly  to  the  central  fact  of  the  origin  of  the  forms 
of  life  by  an  evolutionary  process  that  we  are  compelled 
to  accept  this  deduction,  but  as  to  almost  all  the  essential 
features,  whether  of  cause  or  of  mode,  by  which  specific 
diversity  has  become  what  we  perceive  it  to  be,  we  have 
to  confess  an  ignorance  nearly  total.  [Italics  mine.] 
The  transformation  of  masses  of  population  by  im- 
perceptible steps  guided  by  selection,  is,  as  most  of 
us  now  see,  so  inapplicable  to  the  facts,  whether  of 
variation  or  specificity,  that  we  can  only  marvel  both 
at  the  want  of  penetration  displayed  by  the  advocates 
of  such  a  proposition,  and  at  the  forensic  skill  by 
which  it  was  made  to  appear  acceptable  even  for 
a  time."  2 

These  quotations  might  be  much  extended  and 
diversified,  but  they  cover  the  principal  categories 

1  Gustav  Steinmann:  Die  Abstammungslehre,  Leipzig,  1908,  pp.  1,  2. 

2  William  Bateson:  Problems  of  Genetics,  New  Haven,  1913,  p.  248. 


6  THE  THEORY  OF  EVOLUTION 

of  modern  biological  opinions,  excepting  that  of  the 
strictly  Darwinian  school.  They  suffice,  however,  to 
show  how  little  support  such  opinions  offer  to  those 
who  would  cast  aside  the  theory  of  evolution  as  a 
worn-out  hypothesis.  It  is  clear  that  the  differences 
of  belief  are  almost  wholly  concerned  with  the 
problems  of  modes  and  factors,  not  with  the  theory 
itself.  The  duality  of  meaning  attached  to  the  term 
"Darwinism"  is  largely  responsible  for  the  popular 
misapprehension  as  to  the  present  status  of  the  theory 
among  naturalists.  Darwinism  is  frequently  em- 
ployed as  synonymous  with  evolution,  but  this  is  an 
inexact  usage,  and  the  term  should  be  restricted  to 
Darwin's  explanation  of  the  evolutionary  process. 
Owing  to  this  inaccurate  use  of  the  word,  many 
people,  when  they  hear  that  such  and  such  prominent 
biologists  have  rejected  and  attacked  Darwinism, 
jump  to  the  entirely  false  conclusion  that  the  attack 
is  directed  against  the  theory  of  development. 

The  theory  of  organic  evolution  made  its  appear- 
ance very  early  in  the  history  of  Greek  philosophy 
and  reappeared  from  time  to  time  in  Roman  and 
mediaeval  writers,  because  of  the  charm  which  the 
conception  of  uniformity  and  continuity  in  nature 
exercises  over  many  minds.  But  these  early  sugges- 
tions were  merely  speculative;  no  attempt  was  made 
to  support  the  theory  by  evidence,  or  to  gather  a 
systematic  body  of  observed  facts  which  demanded 
explanation.  Each  writer  was  content  to  quote  from 
such  predecessors  as  appealed  to  his  fancy  or  his 


PRESENT  STATUS  OF  THE  QUESTION        7 

judgment,  without  undertaking  to  test  the  conclu- 
sions handed  down  to  him.  Indeed,  until  the  sciences 
of  Zoology  and  Botany  had  been  gradually  built  up 
and  a  great  body  of  observed  facts  had  been  gathered 
and  arranged  in  an  orderly  manner,  no  other  pro- 
cedure was  possible;  this  did  not  take  place  until 
the  end  of  the  18th  and  beginning  of  the  19th  century. 
Even  then,  however,  little  real  progress  was  made; 
scientific  opinion  was  not  ripe  for  such  a  bold  gener- 
alization and  the  teachings  of  Lamarck  attracted  few 
followers,  especially  as  the  overwhelming  authority 
of  Cuvier  was  exerted  against  these  teachings. 

It  is  of  interest  to  note  that  Lamarck  (1744-1829) 
arrived  at  the  evolutionary  conception,  just  as  Dar- 
win did  nearly  half  a  century  later,  through  a  study 
of  the  problem  of  species.  Species  was  first  em- 
ployed as  a  term  in  logic  and  was  given  a  definite 
application  to  animals  and  plants  by  John  Ray  (1628- 
1705),  who  indicated  by  it  a  group  of  organisms  with 
marked  characteristics  in  common  and  freely  inter- 
breeding. The  concept  received  a  more  precise  mean- 
ing and  definition  from  the  great  Swedish  naturalist 
Linnaeus  (1707-1778),  who  devised  the  modern 
scheme  of  the  classification  and  nomenclature  of 
animals  and  plants.  Linnaeus  was  somewhat  incon- 
sistent in  his  expressions  on  the  subject,  but  the 
doctrine  which  was  accepted  as  his  by  nearly  all  of 
his  contemporaries  and  successors  was  that  species 
were  fixed  entities  which  had  been  separately  created. 
The  Linnaean  dogma,  which  prevailed  down  to  the 


8  THE  THEORY  OF  EVOLUTION 

time  of  Darwin,  is  that  "there  are  as  many  species 
as  God  created  in  the  beginning."  Lamarck,  who 
had  accomplished  an  immense  amount  of  systematic 
work  in  zoology  and  botany,  felt  compelled  to  deny 
the  Linnsean  doctrine  and  to  take  up  the  alternative 
explanation  of  evolutionary  descent. 

Lamarck's  best  known  work  is  his  "Philosophic 
Zoblogique,"  which  is  very  abstruse  and  speculative 
and  contains  but  little  evidence  for  his  conclusions, 
other  than  a  reference  to  his  wide  experience  as  a 
systematist,  i.  e.,  a  student  of  classification.  He 
maintained  that  the  species  was  an  artificial  con- 
ception and  that  nature  knew  only  individuals  in 
graduated  steps  of  relationship.  Most  of  the  book 
is  devoted  to  a  consideration  of  the  causes  which 
have  led  to  the  transformation  of  animals  and  the 
conclusion  is  that  adaptation  to  environment  is  the 
most  efficient  factor.  This  adaptation  is  to  be  ex- 
plained by  the  fact  that  the  'conditions  of  life  deter- 
mine the  manner  in  which  animals  make  use  of  their 
organs;  these  are  developed  and  perfected  by  use, 
but  dwindle  and  are  atrophied  through  disuse.  An- 
imals which  live  in  the  dark,  as  in  caves  or  in  the 
abysses  of  the  ocean,  are  blind,  while  their  organs 
of  feeling  are  highly  developed  to  take  the  place  of 
sight.  As  the  environment  is  subject  to  constant 
change,  adaptation  requires  that  living  beings  should 
be  correspondingly,  though  gradually,  transformed. 

Modern  accounts  of  Lamarck's  views  are  almost 
always  taken  from  the  "Zoological  Philosophy,"  but, 


PRESENT  STATUS  OF  THE  QUESTION        9 

as  Giard  has  pointed  out,  a  clearer  conception  may 
be  obtained  from  an  examination  of  the  successive 
introductory  lectures  of  the  courses  in  zoology  at  the 
Paris  Museum.  Giard  says :  "  It  was  the  attentive  and 
minute  study  of  the  innumerable  species  of  plants 
which  Lamarck,  the  botanist,  had  had  to  describe 
and  classify  in  the  Flore  francaise  and  Encyclopedic 
methodique;  it  was  the  necessity  of  beginning,  at  the 
age  of  fifty,  a  work  of  the  same  kind  for  the  lower  an- 
imals .  .  .  ;  it  was  finally  the  need  of  crowning  with 
a  synthesis  thirty  years  of  analytical  labours,  which 
led  the  great  naturalist,  till  then  a  partisan  of  the 
stability  of  the  species,  to  demonstrate  its  variability 
and  to  search  for  the  causes  of  the  transformation  of 
types."  He  then  gives  an  extensive  quotation  from 
the  introductory  lecture  of  1806,  from  which  the 
following  passage  may  be  taken: 

"All  the  observations  which  I  have  gathered  on 
this  important  subject,  even  the  difficulty  which  I 
know,  from  my  own  experience,  is  now  felt  in  dis- 
tinguishing species  in  genera  where  we  are  already 
very  rich,  a  difficulty  which  is  daily  increased  as  the 
researches  of  naturalists  enlarge  our  collections,  every- 
thing has  convinced  me  that  our  species  have  but  a 
limited  existence  and,  for  the  most  part,  differ  from 
neighbouring  ones  only  by  shades  which  are  difficult 
to  express.  Those  who  have  observed  much  and 
have  examined  great  collections,  have  been  able  to 
convince  themselves  that,  as  the  circumstances  of 
habitat,  of  exposure,  of  climate,  of  nourishment,  of 


10  THE  THEORY  OF  EVOLUTION 

manner  of  life,  are  changed,  the  characters  of  stature, 
form,  proportion  of  parts,  colour,  consistency,  dura- 
tion, agility  and  industry  in  animals  are  proportion- 
ally changed. 

"They  could  see  that  in  animals  the  more  frequent 
and  sustained  use  of  any  organ  gradually  strengthens 
and  develops  that  organ,  enlarges  it  and  gives  it  a 
power  in  proportion  to  the  duration  of  that  use; 
while  constant  disuse  of  such  an  organ  insensibly 
weakens  it,  deteriorates  it,  progressively  diminishes 
its  capacities,  tends  to  destroy  it. 

"Finally,  they  could  note  that  everything  which 
nature  causes  individuals  to  acquire  or  to  lose  through 
the  sustained  influence  of  circumstances  to  which  the 
race  has  long  been  exposed,  she  transmits  by  genera- 
tion to  the  new  individuals  which  spring  from  them. 
These  truths  are  constant  and  cannot  be  mistaken 
save  by  those  who  have  never  observed  and  followed 
nature  in  her  operations."  1 

Lamarck,  as  already  noted,  had  but  few  disciples 
in  his  own  or  immediately  succeeding  times,  largely 
because  of  the  opposition  of  Cuvier,  who  wielded  a 
despotic  authority  over  scientific  opinion  in  France. 
In  Germany  also  the  evolutionary  theory  fell  into 
complete  discredit,  but  for  another  reason;  in  that 
country  the  theory  had  been  advocated  chiefly  by 
the  so-called  "Natural  Philosophers,"  whose  wild 
and  fantastic  speculations  finally  disgusted  sober- 
minded  men  and  turned  their  labors  into  more  ob- 

1  Alfred  Giard:  Controversies  Transformistes,  Paris,  1904,  pp.  13,  14. 


PRESENT  STATUS  OF  THE  QUESTION      11 

jective  channels.  The  Linnaean  doctrine  of  the 
fixity  and  immutability  of  species  held  the  field  so 
exclusively  for  the  next  half  century,  that  Darwin 
tells  us  that  he  had  never  met  and  conversed  with 
any  naturalist  who  did  not  hold  such  a  belief. 

The  way  for  the  great  revolution  effected  by  Dar- 
win was  prepared  indirectly  and  in  a  science  which 
seemed  to  be  very  remote  from  the  field  of  contro- 
versy, viz.,  Geology,  and  the  pioneer  who  opened  this 
new  road  was  Sir  Charles  Lyell  (1797-1875).  Before 
Lyell  began  his  great  work,  the  interpretation  of  the 
earth's  history  was  under  the  sway  of  Cuvier's  theory 
of  "Catastrophism."  According  to  this  theory,  the 
history  of  the  earth  consisted  of  long  periods  of 
tranquil  geographical  development,  interrupted  at 
intervals  by  utterly  indescribable  and  unimaginable 
cataclysms,  when  elemental  forces  were  unchained, 
devastating  the  earth,  breaking  up  and  displacing 
the  rocks  and  destroying  every  living  thing;  of  these 
periods  of  destruction,  d'Orbigny  enumerated  27. 
When  tranquillity  was  restored,  renewed  acts  of  crea- 
tion supplied  animals  and  plants  for  the  depopulated 
earth  and  thus,  as  it  were,  the  curtain  rose  upon  a 
new  scene,  with  a  new  set  of  actors,  the  old  having 
been  completely  swept  away.  It  was  observed  that 
each  fresh  creation  produced  animals  and  plants  of 
a  higher  type,  successively  approximating  the  present 
order  of  things.  Cuvier  was  too  cautious  and  too 
critical  to  declare  dogmatically  that  there  had  ac- 
tually been  many  successive  acts  of  creation  and  he 


12  THE  THEORY  OF  EVOLUTION 

suggested  alternative  hypotheses  to  explain  the  ob- 
served facts.  On  the  other  hand,  he  seems  to  have 
himself  believed  in  the  creations  and  his  contem- 
poraries and  successors  accepted  that  doctrine  as 
his. 

Lyell  reverted  to  the  neglected  teachings  of  the 
Scotchman,  James  Hutton  (1728-1797),  that  the  key 
to  the  past  history  of  the  earth  was  to  be  found  in 
the  work  of  agencies  which  are  now  in  operation. 
Like  Hutton,  he  would  not  admit  that  any  process 
which  is  not  still  at  work  could  be  called  upon  to 
explain  events  in  the  past  and  he  even  went  so  far  as 
to  make  these  existing  processes  the  measure  and 
standard  of  former  operations.  In  the  sharpest  con- 
trast to  Cuvier's  Catastrophism,  LyelPs  theory  was 
that  of  Uniformitarianism,  which  insisted  upon  the 
uniformity  and  complete  continuity  of  the  earth's 
history  and  of  the  agencies  which  had  wrought  such 
profound  changes  upon  and  within  the  globe.  In  the 
early  editions  of  his  famous  "Principles  of  Geology," 
Lyell  was  evidently  on  the  point  of  accepting  the 
theory  of  organic  evolution,  but  later  he  receded 
from  this  position,  because  he  was  not  satisfied  with 
the  evidence  for  it.  Lyell 's  influence  upon  geology 
was  rapid  and  profound  in  England  and  Darwin,  who 
became  his  intimate  friend,  was  conspicuously  under 
that  influence,  as  he  himself  repeatedly  testified. 
One  might  almost  say  that  Darwin's  work  largely 
consisted  in  the  application  of  Lyell's  principle  to 
the  world  of  living  beings. 


PRESENT  STATUS  OF  THE  QUESTION      13 

Unquestionably,  the  most  important  and  influen- 
tial figure  in  the  history  of  the  evolutionary  theory 
is  that  of  Charles  Darwin  (1809-1882),  who  effected 
one  of  the  most  remarkable  and  far-reaching  revolu- 
tions in  all  departments  of  human  thought  of  which 
we  have  any  record.  By  a  curious  coincidence,  Dar- 
win and  Alfred  Russell  Wallace  (1823-1913)  inde- 
pendently reached  the  same  solution  of  the  problem 
concerning  the  origin  of  new  species,  but,  with  a 
fine  absence  of  jealousy  and  self-seeking,  none  too 
common  in  the  history  of  science,  they  cooperated 
in  the  production  of  a  preliminary  paper,  which  ap- 
peared in  1858;  this,  however,  attracted  little  at- 
tention. 

The  publication  of  Darwin's  "Origin  of  Species" 
in  1859  is  one  of  the  great  landmarks  in  the  history 
of  science  and  brought  about,  though  gradually,  an 
extraordinary  revolution  in  opinion.  That  Darwin's 
work  did  not  share  the  fate  of  that  of  Lamarck  and 
other  early  evolutionists  was  due  in  part  to  the 
preparatory  labours  of  Lyell,  who  had  familiarized 
men's  minds  with  the  conception  of  uniformity, 
continuity  and  orderly  progress  in  the  development 
of  the  earth,  but  chiefly  to  Darwin's  method  of 
gathering  and  presenting  the  evidence.  Though 
greeted  at  first  with  a  storm  of  criticism,  not  un- 
mingled  with  abuse,  the  sheer  weight  of  evidence, 
presented  with  such  transparent  simplicity  and 
candour,  inevitably  forced  conviction  even  upon  un- 
willing minds.  But  here  it  is  again  necessary  to 


14  THE  THEORY  OF  EVOLUTION 

emphasize  the  distinction  between  the  two  aspects 
of  Darwin's  work.  His  argument,  supported  by  the 
amazing  mass  of  proof,  which  had  been  gathered 
through  twenty  laborious  years,  gradually  con- 
vinced scientific  men  that  the  theory  of  evolution 
offered  by  far  the  most  probable  and  satisfactory 
solution  of  the  problems  of  the  living  world.  On 
the  other  hand,  Darwin's  explanation  of  the  evolu- 
tionary process,  commonly  known  as  the  theory  of 
"Natural  Selection,"  won  no  such  universal  accept- 
ance and  has  been  a  subject  of  controversy,  more 
or  less  acrimonious,  from  the  time  of  its  first  pro- 
mulgation to  the  present  day.  On  the  other  hand,  it 
must  be  recognized  that  Natural  Selection  played 
a  very  important  part  in  bringing  about  the  general 
acceptance  of  evolution,  for  it  was  the  first  sugges- 
tion of  a  reasonable  and  intelligible  factor  in  pro- 
ducing the  transformation  of  species.  Huxley's 
account  of  his  own  attitude  of  mind  is  well  adapted 
to  explain  this.  He  tells  us  that  before  1858,  he 
had  no  definite  opinions  on  the  subject  and  this 
for  two  reasons:  "firstly,  that  up  to  that  time,  the 
evidence  in  favour  of  transmutation  was  wholly 
insufficient;  and,  secondly,  that  no  suggestion  re- 
specting the  causes  of  the  transmutation  assumed, 
which  had  been  made,  was  in  any  way  adequate 
to  explain  the  phenomena."  l  Huxley  was  never 
fully  satisfied  with  the  doctrine  of  natural  selection, 
but,  nevertheless,  that  doctrine  had  a  large  share 

1  Life  and  Letters,  Vol.  II. 


PRESENT  STATUS  OF  THE  QUESTION      15 

in  making  him  the  convinced  and  thoroughgoing 
evolutionist  that  he  was. 

It  is  interesting  to  retrace  some  of  the  more  signifi- 
cant steps  by  which  Darwin  reached  his  conclusions, 
a  course  which  he  has  made  easy  by  his  autobiograph- 
ical sketch.  The  famous  voyage  in  the  naval  brig 
Beagle,  which  lasted  nearly  five  years  (1831-1836) 
was,  he  tells  us,  "by  far  the  most  important  event 
in  my  life  and  has  determined  my  whole  career." 
Much  of  this  period  was  spent  in  long  journeys  in 
the  interior  of  South  America,  and  the  wealth  of 
observations  thus  gathered  forced  upon  him  the 
problem  of  transmutation.  There  were  three  classes 
of  facts  by  which  he  was  deeply  impressed.  (1)  In 
the  surface  deposits  of  the  Argentine  pampas  he 
found  the  remains  of  gigantic  extinct  animals,  dif- 
fering from,  yet  fundamentally  resembling  those 
which  still  inhabit  the  same  region.  (2)  "The 
manner  in  which  closely  allied  animals  replace  one 
another  in  proceeding  southward  over  the  conti- 
nent." Such  a  succession  of  closely  allied  forms 
is  what  would  be  expected  on  the  evolutionary 
theory;  a  single  species  in  ranging  over  a  very  wide 
area  would  become  modified  so  as  to  give  rise  to 
several  other  species,  while  from  the  doctrine  of 
special  creation  no  explanation  is  possible.  (3) 
What  had  the  greatest  weight  to  his  mind  was  af- 
forded by  his  observations  in  the  Galapagos  archi- 
pelago, a  group  of  volcanic  islands  500  miles  west 
of  the  Ecuador  coast.  As  a  whole,  the  animals  and 


16  THE  THEORY  OF  EVOLUTION 

plants  of  these  islands  are  of  distinctly  American 
type,  yet  most  of  the  species  are  peculiar  to  the 
group  and  each  island  has  a  surprising  number  of 
species  peculiar  to  itself.  "  Reviewing  the  facts  here 
given,  one  is  astonished  at  the  amount  of  creative 
force — if  such  an  expression  may  be  used — displayed 
on  these  small,  barren,  rocky  islands;  and  still  more 
so  at  its  diverse  and  yet  analogous  action  on  points 
so  near  each  other." 

The  growth  of  his  belief  in  evolution  is  thus  de- 
scribed by  Darwin  in  a  letter  written  in  1877:  "When 
I  was  on  board  the  'Beagle,'  I  believed  in  the  per- 
manence of  species,  but,  as  far  as  I  can  remember, 
vague  doubts  occasionally  flitted  across  my  mind. 
On  my  return  home  in  the  autumn  of  1836,  I  im- 
mediately began  to  prepare  my  journal  for  publica- 
tion, and  then  saw  how  many  facts  indicated  the 
common  descent  of  species.  .  .  .  But  I  did  not 
become  convinced  that  species  were  mutable  until 
I  think  two  or  three  years  had  elapsed."  Even  after 
he  had  gained  his  conviction  of  the  mutability  of 
species,  he  could  imagine  no  adequate  cause  of  the 
change  and,  consciously  adopting  the  methods  which 
Lyell  had  used  in  Geology,  he  made  a  great  collec- 
tion of  facts,  gathered  from  breeders,  fanciers,  gar- 
deners, as  well  as  from  his  own  observations,  bearing 
on  the  changes  and  modifications  which  men  had 
brought  about  in  the  various  races  and  breeds  of 
domesticated  animals  and  plants.  "I  soon  perceived 
that  selection  was  the  key-stone  of  man's  success 


PRESENT  STATUS  OF  THE  QUESTION      17 

in  making  useful  races  of  animals  and  plants.  But 
how  selection  could  be  applied  to  organisms  living 
in  a  state  of  nature  remained  for  some  time  a  mystery 
to  me."  The  key  to  this  mystery  was  suggested  to 
him  by  reading  Malthus'  famous  essay  on  Population. 
In  his  own  words:  "Being  well  prepared  to  appre- 
ciate the  struggle  for  existence  which  everywhere 
goes  on  from  long  continued  observation  of  the 
habits  of  animals  and  plants,  it  at  once  struck  me 
that  under  these  circumstances  favourable  variations 
would  tend  to  be  preserved,  and  unfavourable  ones 
to  be  destroyed.  The  result  of  this  would  be  the 
formation  of  new  species.  Here  then  I  had  a  theory 
by  which  to  work." 

Time  fails  to  trace  farther  the  stages  of  growth 
in  Darwin's  opinions.  All  that  can  be  here  at- 
tempted is  a  brief  outline  of  the  theory  which,  after 
nearly  twenty  strenuous  years,  he  gave  to  the  world 
in  the  "Origin  of  Species,"  published  in  1859.  Un- 
like most  other  explanations  of  evolution,  Darwin's 
theory  is  a  chain  of  reasoning,  every  link  of  which  is 
an  undisputed  fact,  the  only  question  being,  does 
the  theory  afford  a  sufficient  and  adequate  explana- 
tion of  the  phenomena?  The  attempts  to  give  a 
definite  and  conclusive  answer,  affirmative  or  nega- 
tive, to  this  question  have  given  rise  to  the  many 
debates  of  which  we  have  by  no  means  heard  the 
last  word. 

(1)  The  first  of  the  undisputed  facts  upon  which 
Darwin's  theory  is  founded  is  that  of  variability.  It  is 


18  THE  THEORY  OF  EVOLUTION 

a  matter  of  the  commonest  observation  that  no  two 
individuals  of  a  species  are  exactly  alike  and,  when 
a  very  large  number  of  specimens  are  studied,  the 
range  of  variation  is  often  found  to  be  exceedingly 
wide.  Such  variations  are  fluctuating  and  repeated 
in  every  generation. 

(2)  The  second  fact  is  the  struggle  for  existence. 
Every  species  in  a  state  of  nature  produces  vastly 
more  young  than  can  possibly  survive  to  maturity; 
if  every  egg  of  the  herring  should  develop  to  an 
adult  fish  and  reproduce  in  its  turn,  it  would  not  be 
long  before  the  Atlantic  Ocean  would  fail  to  contain 
them.     A  statistical  study  of  many  species  leads 
to  the  conclusion  that,  so  long  as  the  conditions  of 
life  remain  the  same,  the  number  of  individuals  of 
a  given  species  is  a  substantially  constant  quantity, 
subject  of  course  to  fluctuations  of  increase  and  de- 
crease.   Thus,  for  every  young  animal  or  plant  that 
reaches  maturity,  an  old  one  must  die  to  make  place 
for  it.    The  struggle  for  existence  is  not  to  be  thought 
of  as  a  combat  to  slay  and  devour,  but  as  a  competi- 
tion for  food,  light  and  the  other  necessities  of  life, 
as  well  as  resistance  or  escape  from  the  attacks  of 
predatory  enemies,  parasites,  diseases  and  the  like. 
Not  only  is  there  intense  competition  between  mem- 
bers of  the  same  species,  but  also  between  allied 
species  with  similar  habits  and  needs. 

(3)  The  third  fact  is  natural  selection,  or,  in  Herbert 
Spencer's  happier  phrase,  the  survival  of  the  fittest. 
Survival  to  the  age  of  reproduction  is  not  a  matter 


PRESENT  STATUS  OF  THE  QUESTION      19 

of  chance,  but  those  individuals  persist  which  are 
the  most  vigorous  and  any  variation  from  the  average 
of  the  species  which  confers  an  advantage  in  the 
struggle  will  tend  to  be  preserved  and  to  increase 
in  successive  generations.  Darwin,  as  he  tells  us, 
very  early  perceived  that  in  making  new  breeds  of 
domesticated  animals  and  plants,  selection  of  the 
parents  according  to  the  characteristics  which  it  was 
desired  to  perpetuate  and  improve,  played  the  im- 
portant role  and  he  concluded  that  advantageous 
variations,  by  enabling  their  possessors  to  survive 
in  the  struggle,  would  furnish  the  material  for  the 
formation  of  new  species,  while  any  disadvantageous 
variation  would  be  promptly  extinguished. 

(4)  Finally,  there  is  the  fact  of  heredity.  It  is  a 
commonplace  that  individuals  are  like  their  parents 
and  should  the  parents  have  from  birth  some  ad- 
vantageous peculiarity,  this  would  tend  to  reappear 
in  the  offspring.  A  repetition  of  this  process  from 
generation  to  generation  would  have  a  cumulative 
effect,  resulting  in  very  extensive  changes. 

While  Darwin  always  maintained  that  natural 
selection  was  by  far  the  most  important  and  effi- 
cient cause  in  producing  transmutation,  he  admitted 
the  action  of  other,  though  minor  factors.  Thus, 
he  makes  frequent  appeal  to  the  effects  of  use  and 
disuse  of  organs,  the  agency  which  Lamarck  had 
regarded  as  of  such  primary  importance,  and  he 
advanced  the  theory  of  sexual  selection  to  account 
for  the  brilliant  colouring  and  elaborate  patterns 


20  THE  THEORY  OF  EVOLUTION 

of  very  many  animals,  especially  of  butterflies  and 
birds.  Inasmuch,  however,  as  the  effects  of  use 
and  disuse  as  an  evolutionary  factor  involve  the 
assumption  that  characters  acquired  during  the 
lifetime  of  the  parent  are  or  may  be  transmitted  to 
the  offspring,  an  assumption  which  is  now  vigorously 
denied,  modern  disciples  of  Darwin  reject  this  factor 
and  attribute  little  importance  to  sexual  selection. 
They  accept  Weismann's  dictum  that  natural  se- 
lection is  the  sole  and  all-powerful  agent  of  evolu- 
tion. 

Darwin's  book,  reenforced  by  his  subsequent 
works,  brought  about  the  astonishing  revolution 
in  scientific  opinion  which  has  already  been  de- 
scribed, but,  while  the  evolutionary  theory  thus 
gained  a  complete  and  almost  universal  victory, 
Darwin's  particular  theory,  that  of  natural  selection, 
was  not  so  fortunate.  True,  it  was  and  still  is  very 
widely  accepted,  but  there  has  always  been  a  large 
body  of  opinion  which  rejected  it  as  vague,  inade- 
quate and  unsatisfactory  and  there  have  been  many 
attempts  to  supplement  it,  or  to  substitute  some 
more  convincing  explanation  for  it.  Thus,  the 
eminent  German  botanist  Carl  von  Nageli  (1817- 
1891)  propounded  an  elaborate  theory  of  develop- 
ment, in  which  he  attempted  to  show  that  natural 
selection  was  an  insufficient  explanation  of  evolution 
and  that  a  "principle  of  perfection"  must  be  as- 
sumed, which  drives  organisms  to  take  on  higher 
and  higher  forms.  He  compares  the  living  world 


PRESENT  STATUS  OF  THE  QUESTION      21 

to  a  garden  full  of  shrubs,  which  are  continually 
sending  out  new  branches,  the  tips  of  which  repre- 
sent the  plants  and  animals  now  existing;  they  grow 
by  their  own  internal  force,  but  are  kept  within 
limits  by  the  action  of  natural  selection,  which 
plays  the  part  of  the  gardener  with  his  pruning- 
shears.  Were  it  not  for  this  pruning,  the  shrubbery 
would  speedily  degenerate  into  a  wild  and  formless 
thicket. 

One  of  the  earliest  and  most  obvious  objections 
made  against  Darwin's  theory  was  that  a  slight 
favourable  variation,  arising  in  a  few  individuals 
would  be  speedily  swamped  by  cross-breeding  with 
the  vastly  more  numerous  individuals  which  did 
not  display  that  variation.  To  avoid  this  objection, 
Moritz  Wagner  propounded  his  theory  of  the  "origin 
of  species  by  separation  in  space,"  according  to 
which  geographical  separation  of  groups  of  indi- 
viduals, by  the  prevention  of  interbreeding,  pro- 
duced new  forms  through  a  process  of  divergent 
development.  Everything  that  we  know  indicates 
that  geographical  separation  has  been  an  important 
factor  in  bringing  about  the  diversity  which  char- 
acterizes the  living  world,  but  few  would  attribute 
to  this  factor  the  significance  which  Wagner  gave 
to  it. 

Professor  August  Weismann  of  Freiburg  (1834- 
1914)  was  a  most  influential  figure  in  the  biological 
controversies  which  raged  in  the  latter  part  of  the 
nineteenth  century,  especially  by  his  elaborate  the- 


22  THE  THEORY  OF  EVOLUTION 

ories  of  heredity  and  the  constitution  of  the  germ- 
plasm,  which  have  been  remarkably  confirmed  by  the 
extensive  studies  which  have  grown  out  of  the  re- 
searches in  the  exact  mechanism  of  heredity  made  by 
Mendel.  Weismann  was  the  first  writer  who  strongly 
questioned  the  hereditary  transmission  of  charac- 
teristics which  had  been  acquired  during  the  life- 
time of  the  parent,  maintaining  that  all  new  struc- 
tures must  first  arise  in  the  germ-plasm.  He  thus 
rejected  altogether  the  effects  of  the  use  and  disuse 
of  organs,  which  Darwin  had  admitted  and  to  which 
Lamarck  had  attributed  the  primary  role  in  the 
work  of  transformation,  and  regarded  natural  selec- 
tion alone  as  the  all-important  factor.  It  would 
require  far  more  time  than  is  at  our  disposal  to  give 
even  an  outline  sketch  of  Weismann's  theories  and 
it  must  suffice  to  point  out  that  his  views  are  still 
largely  in  the  ascendant  and  that  the  great  number 
of  naturalists  who  constitute  the  so-called  "Neo- 
Darwinian  School"  look  upon  Weismann  as  their 
foremost  spokesman. 

Palaeontologists,  whose  studies  deal  with  the  fossil 
remains  of  the  ancient  plants  and  animals  which 
once  inhabited  the  earth,  but  are  now  altogether  ex- 
tinct, have  not,  as  a  rule,  been  satisfied  with  the 
theory  of  natural  selection  as  an  adequate  explana- 
tion of  organic  evolution.  It  was  a  palaeontologist, 
von  Waagen,  who,  in  1867,  first  pointed  out  a  dis- 
tinction, which  may  yet  prove  to  be  the  first  step 
toward  the  formulation  of  an  entirely  acceptable 


PRESENT  STATUS  OF  THE  QUESTION      23 

theory.  This  distinction  was  between  variations, 
on  the  one  hand,  and  mutations,  on  the  other.  By 
variations,  Waagen  understood  the  fluctuating  dif- 
ferences between  contemporary  individuals  of  the 
same  species,  differences  which  are  repeated  in  every 
generation  and  are  entirely  inconstant.  Mutations 
are  successive  changes,  which,  however  slight  they 
may  be,  are  yet  constant  in  character,  and  a  series  of 
mutations  forms  successive  connected  steps  of  modi- 
fication in  a  definite  direction.  The  eminent  Dutch 
botanist  de  Vries  adopted  this  conception  and,  with 
certain  changes,  erected  it  into  an  elaborate  theory, 
which  has  formed  the  subject  of  much  controversy. 
De  Vries  points  out  that  Darwin  did  not  distinguish 
clearly  between  the  two  kinds  of  modifications  which 
have,  or  may  have,  furnished  the  starting-points  for 
the  various  races  and  breeds  of  domesticated  animals 
and  plants.  One  kind  is  the  ordinary  difference 
between  individuals  and  the  other  is  the  "sport,"  or 
sudden  and  notable  change  in  one  or  more  charac- 
teristics, a  change  which  is  constant  and  transmis- 
sible to  the  offspring.  An  often  cited  "sport"  is  the 
Ancon  sheep,  a  breed  which  arose  from  a  single 
short-legged  ram,  born  of  normal  parents  in  1791  on 
a  New  Hampshire  farm.  This  ram  transmitted  his 
peculiarities  to  his  descendants  and  soon  a  new  kind 
of  sheep,  popular  because  of  their  inability  to  jump 
fences,  was  established.  Such  a  sudden,  unheralded 
and  transmissible  change  of  structure  de  Vries  calls 
a  mutation  and  he  believes  that  mutation  is  the 


24  THE  THEORY  OF  EVOLUTION 

normal  manner  in  which  new  species  originate  in 
nature  and  that  the  struggle  for  existence  is  not  so 
much  between  individuals  as  a  competition  between 
mutants  or  incipient  species. 

Interesting  and  important  as  the  mutation  theory 
undoubtedly  is,  it  offers  no  explanation  of  the  phe- 
nomena, for  the  cause  of  such  sudden  changes  re- 
mains a  complete  mystery.  The  seat  of  the  change, 
it  can  hardly  be  questioned,  must  be  sought  in  the 
germ-plasm  of  one  or  both  parents,  but  we  have  not 
the  least  inkling  of  how  such  modification  is  brought 
about. 

Finally,  should  be  mentioned  the  work  of  Johann 
Gregor  Mendel  (1822-1882)  an  Austrian  monk,  who 
made  experiments  in  crossing  different  varieties  of 
peas  and  subjected  his  results  to  a  mathematical 
analysis.  His  paper  was  published  in  1866  and  at- 
tracted no  attention,  but  long  after  his  death  it  was 
rediscovered,  his  results  having  been  independently 
and  almost  simultaneously  attained  by  three  botan- 
ists, who  published  them  in  1900,  de  Vries,  von 
Tschermak  in  Austria  and  Correns  in  Germany.  It 
is  pathetic  to  contrast  the  complete  neglect  with 
which  Mendel  was  treated  during  his  lifetime  with 
the  honour  which  is  paid  to  him  now.  His  work  has 
been  developed  into  a  new  branch  of  biological 
science,  called  "Genetics,"  an  exact  analysis  of  the 
mechanism  of  heredity,  and  will  be  more  fully  dealt 
with  in  the  last  lecture  of  the  course.  Mendel's  name 
is  mentioned  here  merely  to  complete  the  list  of  those 


PRESENT  STATUS  OF  THE  QUESTION      25 

who  have  been  the  foremost  pioneers  in  clearing  the 
way  for  the  modern  conception  of  evolution. 

From  the  quotations  which  have  been  read  in  the 
foregoing  part  of  the  lecture,  it  will  be  only  too  evi- 
dent that,  while  naturalists  are  all  but  unanimous 
in  accepting  the  theory  of  evolution  as  an  established 
truth,  there  is  every  possible  divergence  in  their 
views  as  to  the  causes  of  development  and  diversi- 
fication. Could  a  census  of  opinions  be  taken  among 
the  zoologists  and  botanists  of  the  world,  it  is  very 
probable  that  a  substantial  majority  of  Darwinians 
would  be  found.  By  Darwinians  is  to  be  understood 
those  who  are  convinced  that  the  doctrine  of  natural 
selection  is  the  true  explanation  of  organic  evolution. 
Whether  or  not  a  majority  of  all  naturalists  are  to 
be  ranked  with  the  Darwinians,  it  can  hardly  be 
doubted  that  there  are  very  many  more  who  accept 
natural  selection  than  there  are  of  those  who  have 
adopted  any  alternative  view.  Personally,  I  have 
never  been  satisfied  that  Darwin's  explanation  is  the 
rightful  one;  to  one  who  approaches  the  problem 
from  the  study  of  fossils,  the  doctrine  of  natural 
selection  does  not  appear  to  offer  an  adequate  ex- 
planation of  the  observed  facts.  The  doctrine,  in 
its  application  to  concrete  cases,  is  vague,  elastic, 
unconvincing  and  seems  to  leave  the  whole  process 
to  chance.  To  be  sure,  this  difficulty  is,  to  a  great 
extent,  inherent  in  the  nature  of  the  problem,  for 
direct  observation  of  the  long  course  of  evolutionary 
development  is  impossible;  no  one  ever  saw  the  birth 


26  THE  THEORY  OF  EVOLUTION 

of  a  species  and  thus  we  are  shut  up  to  the  drawing  of 
inferences  from  what  may  be  learned  by  comparison 
and  experiment. 

On  the  other  hand,  if  Darwin's  hypothesis  be 
rejected,  there  is,  it  must  be  frankly  admitted,  no 
satisfactory  alternative  to  take  its  place.  Granting 
that  de  Vries  is  right  in  holding  that  species  originate 
by  sudden  mutation,  the  process  is  not  thereby 
explained  and  made  intelligible  and  it  does  not  seem 
likely  that  such  an  explanation  could  ever  be  found, 
though  it  is  rash  to  set  a  limit  to  future  discovery. 
In  short,  while  the  evolutionary  theory  is  buttressed 
by  such  a  mass  of  evidence  that  nearly  all  men  of 
science  are  convinced  of  its  truth,  no  satisfactory 
and  acceptable  explanation  of  its  causation  has  yet 
been  devised.  To  repeat  Prof.  E.  B.  Wilson's  words: 
"Further  discussions  concerning  the  causes  of  evolu- 
tion had  clearly  become  futile  until  a  more  adequate 
and  critical  view  of  existing  genetic  phenomena  had 
been  attained." 


LECTURE  II 

EVIDENCES    FOR    THE    THEORY— CLASSIFICATION, 
DOMESTICATION  AND  COMPARATIVE  ANATOMY 

Before  attempting  to  set  before  you  an  outline 
sketch  of  the  evidence  which  has  so  firmly  convinced 
zoologists  and  botanists  that  the  theory  of  evolution 
is  true,  it  is  necessary  to  begin  with  a  warning  not  to 
expect  too  much;  that  is  to  say,  more  complete  proof 
than,  in  the  nature  of  things,  it  is  possible  to  obtain. 
Evolution  has  been  a  long,  historical  process,  which 
has  gone  on  with  extreme  slowness  through  un- 
imaginable ages  of  time,  when  no  man  was  there  to 
see  and  observe.  Complete  demonstration,  such  as 
may  be  given  for  a  physical  or  chemical  law,  is  there- 
fore not  to  be  expected.  Even  in  so  exact  a  science 
as  astronomy,  the  attempt  to  work  out  historical 
problems  is  beset  with  the  gravest  difficulties.  No 
astronomer,  for  example,  doubts  that  the  solar  sys- 
tem is  a  unit,  which  has  been  developed  from  some 
originally  continuous  body  of  matter,  such  as  a 
nebula,  but  there  is  no  general  agreement  as  to  the 
manner  in  which  this  development  was  brought 
about.  Any  solution  yet  proposed  is  face  to  face 
with  unsolved  problems.  Clearly,  therefore,  the 
evidence  for  organic  evolution  must  be  indirect,  since 
direct  proof  is  unattainable,  yet  it  need  not  be  un- 

27 


28  THE  THEORY  OF  EVOLUTION 

convincing  for  that  reason,  because  in  science,  as 
in  the  practical  affairs  of  life,  probability  must  be 
the  guide,  whenever  certainty  is  not  reached  and 
certainty  is  very  rarely  reached,  if  ever. 

Speaking  of  plants,  Dr.  D.  H.  Scott  says:  "Our 
ideas  of  the  course  of  descent  must  of  necessity  be 
diagrammatic;  the  process,  as  it  actually  went  on, 
during  ages  of  inconceivable  duration,  was  doubtless 
infinitely  too  complex  for  the  mind  to  grasp,  even 
were  the  whole  evidence  lying  open  before  us.  We 
see  an  illustration,  on  a  small  scale,  of  the  complexity 
of  the  problem,  in  the  case  of  domesticated  forms, 
evolved  under  the  influence  of  man.  Though  we 
know  that  our  cultivated  plants,  for  instance,  have 
been  developed  from  wild  species  within  the  human 
period  and  often  within  quite  recent  years,  yet  noth- 
ing is  more  difficult  to  trace,  in  any  given  instance, 
than  the  true  history  of  a  field-crop  or  garden  plant, 
or  even,  in  many  cases,  to  fix  its  origin  with  cer- 
tainty.'* *  Under  the  infinitely  more  complex  con- 
ditions and  longer  time,  in  which  natural  evolution 
is  believed  to  have  taken  place,  it  is  no  reason  for 
surprise  that  the  evidence  should  be  not  only  in- 
direct, but  also  general,  even  sometimes  vague,  rather 
than  specific  in  character.  On  the  other  hand,  what 
lends  great  cogency  to  this  evidence  is  the  fact  that 
several  quite  distinct  and  independent  lines  of  proof 
all  converge  in  a  common  conclusion.  While  per- 
haps no  one  of  these  lines  is  altogether  complete  or 

»  D.  H.  Scott:  Studies  in  Fossil  Botany,  London,  1900,  pp.  524-5. 


EVIDENCES  FOR  THE  THEORY  29 

convincing,  yet  the  harmony  of  all  of  them,  all  point- 
ing in  the  same  direction,  greatly  strengthens  the 
proof  and  raises  the  theory  to  a  very  high  order  of 
probability,  despite  the  many  unanswered  questions 
and  unsolved  problems.  Classification,  comparative 
anatomy,  embryology,  palaeontology,  the  geographi- 
cal distribution  of  animals  and  plants,  all  tell  the 
same  story.  It  is  because  in  every  department  of 
zoology  and  botany  the  evolutionary  hypothesis 
offers  the  simplest,  the  most  rational  and  the  most 
satisfactory  explanation  of  the  observed  facts,  that 
naturalists  accept  it  almost  unanimously. 

In  order  to  explain  the  character  of  the  living  world 
and  what  is  known  of  its  history,  but  two  alternative 
hypotheses  have  been  propounded.  The  first,  called 
the  doctrine  of  Special  Creation,  holds  that  each 
kind  of  animal  and  plant  was  created  in  its  present 
form  and  is  substantially  immutable.  This  doctrine, 
formulated  by  Linnaeus,  was  very  widely  adopted 
and  held  the  field  until  the  publication  of  Darwin's 
book.  The  second  is  the  hypothesis  of  evolution,  or 
"descent  with  modification,"  according  to  which  all 
animals  and  plants,  as  we  know  them,  have  been 
developed  from  primordial  germs,  leaving  open  the 
question  as  to  how  life  on  the  earth  originated.  If 
any  one  declines  to  accept  either  one  of  these  hy- 
potheses, it  only  remains  to  admit  that  the  problem 
is  a  complete  mystery,  for  which  no  solution  has  been 
found,  the  position  taken  by  Fleischmann.  In  mak- 
ing a  choice  between  these  alternatives,  it  often 


80  THE  THEORY  OF  EVOLUTION 

seems  to  be  taken  for  granted  that  the  burden  of 
proof  rests  entirely  upon  the  evolutionary  theory, 
but  this  is  a  mistake,  for  there  is  and  can  be  no  evi- 
dence for  that  of  special  creation.  The  only  thing 
which  could  justify  us  in  holding  that  doctrine  would 
be  to  find  that  the  facts  could  be  explained  in  no 
other  way. 

It  has  been  objected  that  the  gradations  between 
the  various  forms  of  life,  to  which  evolutionists 
attach  so  much  importance,  is  in  itself  no  proof  of  a 
genetic  connection  between  those  forms,  and  the 
gradual  development  of  human  productions  is  ad- 
duced as  showing  similar  relations  without  any 
genetic  connections.  For  example,  houses  may  be 
traced  from  their  earliest  beginning  in  the  rude  hut 
of  the  savage,  through  various  stages  to  the  most 
extensive  and  elaborate  modern  dwelling,  yet  no  one 
would  suggest  that  houses  were  evolved  by  descent 
with  modification.  Here  the  relationship  is  ideal, 
not  material,  and  the  development  is  that  of  a  plan; 
why  should  the  same  thing  not  be  true  of  the  living 
world?  In  reply,  it  may  be  said  that  this  objection 
might  have  some  weight,  if  houses  naturally  repro- 
duced their  kind,  as  all  living  things  do,  and  each 
individual  structure  were  not  the  work  of  men's 
hands.  Then,  too,  in  order  to  hold  the  evolutionary 
hypothesis,  it  is  not  necessary  to  deny  the  ideal  re- 
lationships between  the  successive  gradations  of 
living  beings,  or  to  exclude  belief  in  a  creative 
plan,  which  has  been  worked  out  by  the  method 


EVIDENCE  FROM  CLASSIFICATION        31 

of  evolution.  This,  however,  is  a  metaphysical, 
rather  than  a  scientific  question,  and  has  no  im- 
mediate bearing  upon  the  evidences  for  the  theory 
of  evolution. 

As  has  already  been  pointed  out,  the  evidence  for 
the  theory  consists  of  several  distinct  and  independ- 
ent lines  of  testimony,  so  to  speak,  and  it  will  con- 
duce to  clearness  of  treatment,  if  each  of  these  lines 
is  dealt  with  separately,  though  their  action  and 
reaction  upon  one  another  must  not  be  overlooked. 
In  the  first  place,  we  may  take  up  the  argument  from 
classification,  which  is  the  most  obvious  one  and  has 
the  additional  historic  interest  of  being  the  line  along 
which  Lamarck  and  Darwin  approached  and  reached 
their  primary  generalizations.  It  was,  in  short,  the 
problem  of  species  which  led  those  great  naturalists 
to  abandon  the  current  belief  in  the  fixity  and  im- 
mutability of  living  types  and  to  adopt  the  hypothesis 
of  development.  To  repeat,  in  part,  the  quotation 
from  Lamarck  given  in  the  first  lecture:  "The  diffi- 
culty which  I  know,  from  my  own  experience,  is  now 
felt  in  distinguishing  species  .  .  .  has  convinced  me 
that  our  species  have  but  a  limited  existence  and, 
for  the  most  part,  differ  from  neighbouring  ones  only 
by  shades  which  it  is  difficult  to  express."  (See 
ante,  p.  9.) 

The  object  of  classification  is  to  arrange  the  ex- 
traordinarily manifold  assemblages  of  animals  and 
plants  in  groups  of  ascending  comprehensiveness, 
which  shall  express,  so  nearly  as  the  present  state  of 


32  THE  THEORY  OF  EVOLUTION 

knowledge  permits,  the  degrees  of  relationship  be- 
tween them.  In  this  elaborate  scheme,  which  was 
developed  by  Linnaeus  about  the  middle  of  the 
eighteenth  century,  the  unit,  so  to  speak,  is  the 
species,  the  storm-centre  of  many  controversies.  Lin- 
naeus believed  that  species  were  real,  objective  en- 
tities, which  it  was  the  business  of  the  naturalist  to 
distinguish,  describe  and  name,  and  for  a  very  long 
time  the  principal  occupation  of  zoologists  and  botan- 
ists was  the  discrimination  of  species,  furnishing  them 
with  names  and  arranging  them  in  genera,  families, 
orders  and  other  groups  of  higher  rank.  This  pro- 
cedure may  best  be  explained  by  an  example.  The 
European  wolf  is  a  species,  lupus,  which,  with  other 
typical  wolves,  is  included  in  the  genus  Canis  and  is 
therefore  designated  as  Canis  lupus,  for  the  name  of 
a  species  requires  that  of  its  genus  to  identify  it  and 
hence  the  Linnaean  scheme  of  nomenclature  is  said 
to  be  binomial.  The  true  wolves,  jackals,  foxes, 
(genus  Vulpes)  and  many  other  genera  of  differing 
dog-like  forms  are  grouped  together  in  the  family 
CanidcB,  which,  associated  with  all  the  other  ter- 
restrial beasts  of  prey,  cats,  bears,  raccoons,  weasels, 
otters,  etc.,  etc.,  are  included  in  the  sub-order  Fis- 
sipedia.  The  latter  are  joined  with  the  marine  forms, 
seals,  sea-lions,  walruses,  etc.,  to  constitute  the  order 
Carnivora.  Omitting  certain  intermediate  groups, 
we  find  that  the  order  Carnivora  belongs  in  the  class 
Mammalia,  or  warm-blooded  quadrupeds,  which,  in 
turn,  is  a  member  of  the  phylum  Vertebrata,  or 


EVIDENCE  FROM  CLASSIFICATION        33 

animals  with  internal  skeleton,  which  also  includes 
the  fishes,  amphibians,  reptiles  and  birds. 

The  principle  in  accordance  with  which  these 
groups  of  an  ascending  order  of  comprehensiveness 
are  constructed,  is  simply  the  greater  or  less  degree 
of  likeness  in  structural  characteristics.  All  mam- 
mals agree  in  fundamental  features,  however  much 
they  may  differ  in  size,  form,  appearance  and  habits, 
and  thus  such  different-looking  creatures  as  a  whale, 
a  bat,  a  horse,  a  mole  and  a  monkey  are  all  referred 
to  the  same  class.  Under  the  Linnsean  conception, 
which  prevailed  down  to  Darwin's  time,  the  relation- 
ship between  the  various  species  of  a  genus,  or  be- 
tween the  genera  of  a  family,  the  families  of  an  order, 
etc.,  was  regarded  as  purely  ideal.  If  each  species 
represented  a  distinct  act  of  creation  and  was  so 
fixed  as  to  be  incapable  of  transformation  into  an- 
other species,  then  obviously  no  actual  genetic  re- 
lationship, or  blood-kinship  between  different  species 
was  possible.  According  to  the  evolutionary  hy- 
pothesis, on  the  other  hand,  the  relationship  is  truly 
genetic  and  the  differences  are  due  to  divergent  de- 
velopment, acting  under  different  conditions  and  cir- 
cumstances, while  the  deep-seated,  fundamental  like- 
nesses are  explained  as  inheritances  from  a  common 
ancestry. 

As  Lamarck  so  emphatically  pointed  out,  this  dis- 
tinction of  species  from  one  another  is  often  exceed- 
ingly difficult  and  has  frequently  given  rise  to  great 
differences  of  opinion  and  practice  among  systema- 


34  THE  THEORY  OF  EVOLUTION 

lists,  because  of  the  manner  in  which  the  different 
individuals  of  a  species  vary  among  themselves  and, 
in  many  instances,  of  the  way  in  which  the  species 
of  a  genus  grade  into  each  other.  No  two  individuals, 
belonging  to  the  same  species,  are  exactly  alike  in  all 
respects  and  when  a  very  large  number  of  such  in- 
dividuals are  compared,  they  are  found,  as  a  rule, 
to  cover  a  remarkably  wide  range  of  variation,  the 
extremes  of  which  often  differ  more  than  do  the  in- 
dividuals which  are  referred  to  distinct  species.  The 
species  of  a  large,  thriving  and  vigorous  genus  are 
often  so  unstable  and  so  connected  by  intergrada- 
tions,  that  there  is  sometimes  a  radical  difference  of 
opinion  as  to  how  many  separate  species  should  be 
recognized.  "Nothing  is  brought  out  more  clearly 
by  ecological  studies  in  New  Zealand  than  the  ex- 
treme *  plasticity'  of  many  species  and  structures, 
and  their  rapid  response  to  a  change  of  environment. 
This  is  so  great  in  numerous  instances  that  the  idea  of 
'normal'  loses  its  meaning."  l 

For  example,  the  botanists  differ  greatly  in  regard 
to  the  American  oaks  and  into  how  many  species 
they  are  properly  divisible;  the  same  is  true  of  many 
other  groups.  To  evade  this  difficulty,  at  least  in 
part,  several  groups,  subordinate  to  species,  are  recog- 
nized and  named  in  systematic  botany  and  zoology, 
such  as  geographical  races,  varieties,  subspecies,  etc., 
each  of  these  minor  groups  being  relatively  constant 
and  recognizable,  yet  not  differing  sufficiently  from 

1 L.  Cockayne  in  Trans.  New  Zealand  Institute.  Vol.  XLIV,  pp.  13, 14. 


EVIDENCE  FROM  CLASSIFICATION         35 

the  others  to  require  separation  as  a  distinct  species. 
Yet  if  the  natural  origin  of  these  minor  subdivisions 
be  admitted,  there  can  be  no  valid  reason  for  denying 
the  natural  origin  of  species,  genera  and  the  higher 
groups  of  classification,  for  the  difference  is  one  of 
degree,  not  of  kind,  and  the  intergradation  is  often  so 
perfect,  that  it  is  a  matter  of  personal  judgment  and 
experience,  whether  the  systematist  shall  regard  a 
given  group  of  individuals  as  a  variety,  a  subspecies, 
or  a  species,  whence  the  many  differences  and  con- 
flicts in  practice. 

If  a  species,  say  of  birds,  having  a  very  wide 
geographical  range,  be  carefully  examined  and  com- 
pared, it  very  frequently  happens  that  specimens 
taken  from  the  extreme  points  of  that  range  differ 
so  notably  from  one  another,  that  the  systematist 
would  not  hesitate  to  separate  them  as  distinct 
species,  were  it  not  that  in  the  intervening  area  all 
the  intergradations  between  the  extreme  forms  can 
be  found.  The  recognition  of  species  is  thus  often  a 
purely  arbitrary  procedure,  as  to  which  different 
writers  diverge  widely  in  their  judgment.  All  this 
flux  and  inconstancy  and  intergradation  between 
very  distinct  extremes  are  hard  to  comprehend,  if 
we  adopt  the  hypothesis  of  special  creation,  while, 
on  the  other  hand,  they  are  precisely  what  would 
be  anticipated  under  the  hypothesis  of  evolution. 

A  study  of  the  races  of  domesticated  animals  and 
plants,  which  Darwin  l  investigated  so  thoroughly, 

1  The  Variation  of  Animab  and  Plants  under  Domestication,  2  vols.,  1868. 


36  THE  THEORY  OF  EVOLUTION 

leads  to  a  similar  conclusion.  Under  the  conditions 
of  domestication,  these  creatures  have  been  so  pro- 
foundly modified  that,  in  many  instances,  the  orig- 
inal, wild  progenitor  of  the  race  or  breed  cannot  be 
determined.  The  various  breeds  of  horses,  cattle, 
sheep,  pigeons,  fowls,  not  to  mention  the  garden  and 
field-plants,  differ  among  themselves  far  more  than 
do  many  wild  species  from  each  other.  Were  the 
pointer,  terrier  and  spaniel  dogs,  the  fan-tail,  pouter 
and  tumbler  pigeons  found  in  a  state  of  nature,  no 
zoologist  would  hesitate  for  a  moment  to  recognize 
them  as  well-defined  species,  yet  we  know  that  all  the 
breeds  of  pigeons  have  been  derived  from  a  single 
wild  form.  Dogs  are  all  domesticated  wolves,  but 
the  history  of  the  various  breeds  has  not  been  pre- 
served and  it  is  not  known  whether  all  the  familiar 
races  are  descended  from  a  single  wild  species  or 
from  several,  or  what  that  wild  progenitor  or  pro- 
genitors were.  All  this  diversity  has  been  produced 
by  human  agency,  acting  sometimes  with  intelligent 
and  conscious  purpose,  sometimes  with  no  definite 
end  in  view,  by  taking  advantage  of  variability  and 
individual  differences.  If  man  can  accomplish  such 
results  in  a  relatively  short  time,  it  is  difficult  to  see 
why  nature,  with  illimitable  aeons  at  her  disposal, 
should  not  bring  about  correspondingly  greater  trans- 
formations. The  phenomena  of  domestication  make 
the  dogma  of  the  fixity  and  immutability  of  species 
seem  very  ill-founded  and  improbable.  This  is  a 
very  large  subject  and  it  has  been  necessary,  on  ac- 


EVIDENCE  FROM  DOMESTICATION        37 

count  of  the  limitations  of  time,  to  compress  into  a 
few  sentences,  what  Darwin  has  given  in  two  large 
volumes.  To  these  fascinating  volumes  may  be  re- 
ferred the  reader  who  is  interested  in  following  out 
this  line  of  evidence. 

Even  under  conditions  which  cannot  be  called 
domestication,  certain  remarkable  transformations  of 
animals  have  been  observed.  An  interesting  case  of 
this  sort  is  that  of  the  Porto  Santo  Rabbit,  as  de- 
scribed by  Darwin.  At  a  date  variously  given  as 
1418,  1419  and  1420,  the  Portuguese  navigator  Zarco 
set  free  a  doe  and  newly  born  litter  of  young  rabbits 
upon  the  small  island  of  Porto  Santo,  not  far  from 
Madeira.  From  the  fact  that  the  doe  littered  on  ship- 
board, it  is  evident  that  she  belonged  to  one  of  the 
domestic  races,  all  of  which  have  been  derived  from 
the  European  wild  Rabbit  (Lepus  cuniculus).  The 
absence  of  any  carnivorous  creatures,  which  would 
have  preyed  upon  the  rabbits,  or  of  any  other  land 
mammal,  which  might  have  competed  with  them  for 
food,  led  to  a  very  rapid  multiplication,  so  that  in 
less  than  40  years  they  are  described  as  "innumer- 
able." As  a  result  of  four  and  a  half  centuries  of 
isolation  under  these  novel  conditions,  the  Porto 
Santo  Rabbit  has  become  so  different  from  any  of  the 
domestic  races  and  from  its  wild  progenitor,  that 
Haeckel  has  described  it  as  a  distinct  species,  Lepus 
huxleyi.  The  new  form  is  much  smaller  than  the 
European  wild  Rabbit,  weighing  but  little  more  than 
half  as  much  as  the  latter,  and  differs  considerably  in 


38  THE  THEORY  OF  EVOLUTION 

colour.  The  fur  is  redder,  with  far  fewer  black-tipped 
hairs;  the  throat  and  belly  are  gray  or  lead-coloured, 
instead  of  pure  white,  and  the  tips  of  the  ears  and 
upper  surface  of  the  tail  lack  the  blackish-gray  fur 
so  characteristic  of  the  European  wild  form. 

"The  two  little  Porto  Santo  rabbits,  whilst  alive 
in  the  Zoological  Gardens,  had  a  remarkably  dif- 
ferent appearance  from  the  common  kind.  They 
were  extraordinarily  wild  and  active,  so  that  many 
persons  exclaimed  on  seeing  them  that  they  were 
more  like  large  rats  than  rabbits.  They  were  noc- 
turnal to  an  unusual  degree  in  their  habits,  and  their 
wildness  was  never  in  the  least  subdued.  .  .  .  Lastly, 
and  this  is  a  highly  remarkable  fact,  Mr.  Bartlett 
could  never  succeed  in  getting  these  two  rabbits, 
which  were  both  males,  to  associate  or  breed  with 
the  females  of  several  breeds  which  were  repeatedly 
placed  with  them. 

"If  the  history  of  these  Porto  Santo  rabbits  had 
not  been  known,  most  naturalists,  on  observing  their 
much  reduced  size,  their  colour,  reddish  above  and 
gray  beneath,  their  tails  and  ears  not  tipped  with 
black,  would  have  ranked  them  as  a  distinct  species. 
They  would  have  been  strongly  confirmed  in  this 
view  by  seeing  them  alive  in  the  Zoological  Gardens, 
and  hearing  that  they  refused  to  couple  with  other 
rabbits.  Yet  this  rabbit,  which  there  can  be  little 
doubt  would  thus  have  been  ranked  as  a  distinct 
species,  has  certainly  originated  since  the  year  1420. 
Finally,  from  the  three  cases  of  the  rabbits  which 


EVIDENCE  FROM  DOMESTICATION        39 

have  run  wild  in  Porto  Santo,  Jamaica,  and  the 
Falkland  Islands,  we  see  that  these  animals  do  not, 
under  new  conditions  of  life,  revert  to  or  retain  their 
aboriginal  character,  as  is  so  generally  asserted  to 
be  the  case  by  most  authors."  l 

Another  instance  of  transformation,  in  this  case 
very  rapid,  under  "new  conditions  of  life"  is  that  of 
the  Lunar  Moth  (Saturnia  lund)  when  transported 
from  Texas  to  Switzerland.  In  the  year  1870,  the 
entomologist  Boll  brought  to  Switzerland  a  number 
of  cocoons  of  this  large  and  beautiful  moth  and  in 
May  of  the  following  year  the  full-grown  moths 
emerged  from  the  cocoons  and  differed  in  no  par- 
ticular from  the  ordinary  Texas  form.  From  these 
moths  several  hundred  fertilized  eggs  were  obtained, 
from  which,  in  the  course  of  a  few  weeks,  small  cater- 
pillars were  hatched.  In  Texas  the  caterpillars  of 
this  species  feed  upon  the  leaves  of  hickory  and  black 
walnut  trees,  but,  as  such  leaves  were  not  to  be  had 
in  Switzerland,  those  of  the  European  walnut  were 
substituted.  The  substitution  was  entirely  accept- 
able to  the  caterpillars,  which  ate  greedily  and 
formed  their  cocoons  at  the  end  of  June,  the  adult 
moths  emerging  early  in  August.  Much  to  the  sur- 
prise of  all  observers,  this  second  generation  of 
moths,  the  caterpillars  of  which  were  fed  upon  a  plant 
new  to  them,  differed  so  much  in  form,  colour  and 
markings  from  the  parent  Texan  species,  that  any 


Variation  of  Animals  and  Plants  under  Domestication, 
Vol.  I.  pp.  119-20. 


40  THE  THEORY  OF  EVOLUTION 

entomologist  would  have  regarded  it  as  a  new  and 
distinct  species,  had  its  origin  not  been  known. 
Nevertheless,  Dr.  Gemminger,  a  distinguished  ento- 
mologist, recognized  it  as  a  distinct  species  and 
named  it  "  Saturnia  bolli." 

Moritz  Wagner,  from  whom  this  account  is  taken, 
gives  the  following  description  of  the  new  form:  "At 
the  first  glance,  the  connoisseur  is  surprised  by  the 
striking  change  of  form.  In  the  new  species  the  shape 
of  the  body,  as  of  the  wings,  is  somewhat  larger  and 
heavier,  while  the  feathery  antennae  are  slightly 
narrower  and  less  luxuriant.  From  the  longer  hind- 
body  of  the  new  species  the  carmine-red,  longitudinal 
stripes,  which  the  parent  species  bears,  have  com- 
pletely disappeared.  The  front  wings  have  a  less 
sinuous  form,  but  are  somewhat  broader.  This 
change  of  form  is  to  be  more  decidedly  observed  in 
the  tail-like  prolongation  of  the  hind  wings.  Not 
less  striking  than  the  differences  of  shape  are  those 
of  colour.  In  the  ancestral  species  the  colour  is  a 
yellowish  green,  while  that  of  the  new  species  is  a 
beautiful  lemon-yellow.  The  carmine-red  marginal 
stripe,  with  whitish  inner  border,  borne  on  the  ante- 
rior wings  of  Saturnia  luna,  has  quite  disappeared 
in  Saturnia  bolli  and  is  indicated  only  by  a  very 
narrow,  dark  yellowish  colouring  of  the  outermost 
margin. 

"Most  remarkable  in  this  new  species  is  the  ap- 
pearance of  a  new  marking  on  the  anterior  wings, 
which  appears  as  a  transverse  stripe,  with  somewhat 


EVIDENCE  FROM  DOMESTICATION       41 

zig-zag  outer  edge,  but  is  entirely  absent  from  the 
anterior  wings  of  the  parent  species."  1 

Other  similar,  if  less  striking,  examples  of  change 
might  be  described,  did  time  permit;  all  that  can  be 
attempted  here  is  to  give  an  outline  sketch  of  the 
evidence  with  a  few  illustrations,  chosen  from  the 
many  that  are  available.  To  present  anything  like 
an  exhaustive  display  of  the  evidence  would  require, 
not  one  course  of  lectures,  but  many.  The  instances 
selected  suffice  to  show  that,  when  placed  under  new 
conditions  of  climate,  food-supply,  attacks  of  enemies 
and  the  like,  animals  and  plants,  whether  domes- 
ticated or  free,  may  experience  very  marked  changes 
of  size,  form  and  appearance  and  that  species  are, 
in  very  many  cases,  if  not  in  all,  far  from  immutable. 
Often  the  metamorphosis  is  so  great,  that  the  wild 
progenitor  of  many  domesticated  animals  and  cul- 
tivated plants  cannot  be  determined  with  any  degree 
of  certainty. 

The  other  lines  of  evidence  which  are  relied  upon 
to  prove  the  theory  of  evolution  are  comparative 
anatomy;  embryology,  the  study  of  individual  devel- 
opment; palaeontology,  the  study  of  extinct  plants 
and  animals  which  formerly  existed  on  the  earth;  the 
geographical  distribution  of  organisms,  animal  and 
vegetable;  finally,  experimental  investigation.  Each 
one  of  these  methods  of  research  has  its  particular 
advantages,  as  well  as  its  special  limitations  and 

1  M.  Wagner:  Die  Entstehung  der  Arten  dutch  raumliche  Sonderung, 
pp.  30^-10. 


42  THE  THEORY  OF  EVOLUTION 

drawbacks;  it  is  the  harmonious  result  of  all  of  them, 
pointing  in  the  same  direction,  which  gives  great 
weight  to  their  combined  testimony. 
^€fomparative  anatomy,  as  the  name  implies,  is  the 
comparison  of  structure  and  tissues  in  the  whole 
range  of  animal  life.  Such  comparison  leads  to  the 
conclusion  that  in  the  animal  kingdom  there  are 
several  distinct  types  of  structure,  characterizing  the 
grand  divisions  of  the  kingdom.  Within  any  one  of 
these  grand  divisions  may  be  found  almost  endless 
diversities  of  structure,  all  of  them  obviously  different 
modifications  of  the  same  fundamental  plan.  How- 
ever great  the  modification,  the  identity  of  the  plan  is 
clearly  to  be  perceived  throughout,  just  as  is  the 
theme  of  a  musical  composition,  which  may  be 
traced  through  all  the  variations  into  which  the 
composer  has  woven  it.  A  few  concrete  instances  will 
make  clear  the  application  of  this  principle.  Super- 
ficially examined,  the  following  structures  would 
seem  to  have  little  or  nothing  in  common,  but  rather 
to  be  constructed  on  totally  distinct  plans  for  al- 
together different  purposes:  the  fore  leg  of  a  lizard, 
the  wing  of  a  bird  and  of  a  bat,  the  burrowing  shovel 
of  a  mole,  the  flipper  of  a  whale,  the  fore  leg  of  a 
horse,  the  human  arm  and  hand.  When,  however, 
all  of  these  structures  are  carefully  dissected  and  the 
bones,  muscles  and  nerves  are  compared,  it  becomes 
immediately  apparent  that  the  plan  of  structured 
the  same  throughout,  modified  to  serve  different 
uses,  of  running,  flying,  burrowing,  swimming  and 


: 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  43 

: grasping.  In  all  of  them,  the  upper  arm  has  a  single 
bone,  the  humerus,  and  the  forearm  two,  thejiiaa 
and  radius;  in  all  there  is  a  wrist,  formed  of  an  as- 
semblage of  small  polygonal  bones,  followed  by  the 
long  bones  of  the  hand  and,  finally,  by  the  fingers. 
Modification  is  brought  about  by  the  reduction  and 
even  the  suppression  of  certain  parts,  the  enlarge- 
ment of  other  parts  and,  very  much  more  rarely,  by 
the  addition  of  new  parts.  Compare,  for  example, 
the  human  arm  and  hand  with  the  fore  leg  of  a  horse. 
The  human  hand  is  a  grasping  organ  and  rotates 
\freely  on  the  arm;  there  are  five  digits,  one  of  which, 
the  thumb,  is  opposable  to  the  others,  and  each  digit 
is  tipped  with  a  flat  nail.  The  wrist-joint  is  composed 
of  eight  bones,  arranged  in  two  transverse  rows;  the 
two  bones  of  the  forearm  are  complete,  entirely 
separate  and  movable  on  each  other,  the  power  of 
rotation  of  the  hand  being  dependent  upon  the  char- 
acter of  the  elbow  joint.  The  upper  arm-bone  forms 
a  ball  and  socket  joint  with  the  shoulder  blade  and 
rotates  easily  in  almost  any  direction.  In  the  horse, 
on  the  contrary,  the  fore  limb  is  exclusively  an  organ 
^_  of  locomotion  and  cannot  Be  used  for  grasping;  as  a 
*"  wholel'the  limb  ™^yn  r™Jy  bnrl-mrrl  and  forward  in 
a  plane  parallel  with  that  of  the  backbone  and  each 
of  the  joints  is  movable  only  in  the  same  fore-and-aft 
direction,  there  being  no  power  of  rotating  the  hand 
upon  the  arm,  or  of  the  fore  limb  as  a  whole  upon  the 
shoulder.  There  is  but  a  single  functional  digit,  the 
third  of  the  original  five,  the  bones  of  which  are  so 


44 


THE  THEORY  OF  EVOLUTION 


enlarged  and  strengthened  as  to  support  the  whole 
weight  of  the  body;  thejiajLis-greatly  expanded^-to— 
form  a  hoof,  which  encloses  as  in  a  box  the  whole 


fcu 


m.  a.  2X.    m. 

FIG.  1.  Right  fore  limbs  of  Man  (left);  Horse  (middle);  Ox  (right). 
H.  —  humerus.  R.  =  radius.  U.  —  ulna.  C.  =  carpus.  Me.  = 
metacarpals.  Digits  are  numbered  with  Roman  numerals. 

terminal  joint  of  the  digit.  Beside  the  single  func- 
tional digit,  there  are  remnants,  orjrudiments.oi  two 
others,  the  second  and  fourth  of  the  original  series, 
which  are  not  visible  externally  and  consist  only  of 
the  long  bones  of  the  hand  (metacarpals)  without 
free  joints  or  hoofs,  and  are  commonly  known  as 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  45 

"splint-bones."  The  wrist  has  seven  bones,  one  of 
the  lower  row  being  suppressed  as  needless,  in  corre- 
lation with  the  reduction  in  the  number  of  digits. 
One  of  the  forearm  bones,  the  ulna,  is  very  greatly 
reduced,  so  that  only  the  two  ends  remain  and  these 
are  firmly  co-ossified  with  the  radium,  which  is  cor- 
respondingly enlarged  and  bears  the  whole  body- 
weight. 

The  fore  leg  of  the  ox,  like  that  of  the  horse,  is 
exclusively  locomotive  in  function  and  can  move 
only  backward  and  forward,  there  being  no  power  of 
rotation  of  the  hand  on  the  forearm  or  of  the  whole 
leg  on  the  shoulder.  Jn  the  forearm  the  enlarged 
radius  carries  all  the  weight  and  the  two  ends  of  the 
reduced  ulna  are  co-ossified  with  it.  The  most 
striking  difference  from  the  horse  is  to  be  found  in  the 
fact  that  the  foot  has  two  functional  digits,  the  third 
and  fourth,  between  which  the  weight  is  equally 
divided,  and  the  two  hoofs  are  so  shaped  as  to  have  a 
split  appearance,  which  is  expressed  in  the  term 
"cloven-hoofed."  While  accurately  descriptive  of 
the  appearance,  this  term  is  erroneous  if  taken  to 
mean  that  an  originally  single  hoof  has  been  divided. 
The  long  bones  of  the  hand,  or  metacarpals,  are 
completely  fused  together  into  a  compound  "cannon 
bone"  and,  in  addition  to  the  two  functional  digits, 
there  are  two  rudimentary,  externally  visible  ones, 
the  "dew-claws,"  which  are  small,  nodular  hoofs  be- 
hind the  principal  ones.  In  the  horse  and  ox,  then, 
we  see  two  slightly  divergent  modes  of  converting 


46  THE  THEORY  OF  EVOLUTION 

the  primitive,  five-toed  and  mobile  anterior  limb  into 
structures  which  are  purely  locomotive  in  function. 

It  is  evident  that  the  same  elements  are  present 
in  the  human  hand  and  arm  as  in  the  fore  leg  of  the 
horse  and  ox,  but  in  each  case  characteristically 
modified  to  serve  different  ends.  Man's  hand,  for- 
tunately for  us,  is  really  a  very  primitive  and  un- 
differentiated  structure  and  can  be  put  to  a  great 
many  different  uses.  Had  it  been  highly  specialized 
for  a  single  purpose,  human  progress  and  civilization 
would  have  been  impossible,  for  these  have  always 
depended  upon  the  co-ordination  of  hand,  eye  and 
brain.  In  the  horse  everything  has  been  sacrificed 
to  speed,  making  the  animal  a  "cursorial  machine," 
and,  to  this  end,  the  functional  digits  have  been 
reduced  from  five  to  one,  the  proportionate  lengths 
of  the  various  segments  of  the  limb  adjusted  to  the 
best  advantage  and  the  feet  raised  from  the  ground, 
so  that  the  horse,  as  also  the  ox,  walks  upon  the  very 
tips  of  the  toes.  What  is  ordinarily  called  the  * '  knee ' ' 
of  a  horse  is  the  exact  homologue  of  the  human  wrist, 
while  the  elbow  joint  is  concealed  in  the  muscles  of 
the  shoulder. 

Again,  the  flipper  of  a  whale  is,  to  all  external  ap- 
pearances, totally  different  from  the  human  arm 
and  hand,  as  from  the  fore  leg  of  the  horse,  but  dis- 
section shows  that  the  plan  of  structure  is  the  same, 
identical  parts  being  greatly  modified  in  form  and 
function  in  accordance  with  the  exclusively  marine 
life  of  the  animal.  Except  at  the  shoulder,  there  is 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  4? 


no  true  joint  between  any  of  the  bones,  which  ar- 
rangement allows  but  a  very  limited  motion,  merely 
enough  for  the  necessary  flexibility  of  the  paddle. 
The  arm-bones  are  short,  mas- 
sive and  simple  and,  what  is 
extraordinarily  rare  among  mam- 
mals, the  number  of  joints  in 
the  digits  is  much  increased, 
to  give  the  needed  length 
and  support  to  the  flipper. 
Clearly,  the  whale's  paddle  is  of 
precisely  the  same  type  as  the 
fore  limb  of  a  land  quadruped, 
but  transformed  and  adapted  to 
a  purely  marine  habit  of  life. 

The  comparison  of  a  bat's 
wing  with  that  of  a  bird  brings 
to  light  two  quite  different  modes 
of  adaptation  of  similar  elements 
to  the  purposes  of  flight,  and  in 
both  groups  we  find  certain  dif- 
ferences in  the  completeness  of 
adaptation.  In  the  bat  the 
bones  of  the  arm  are  very  slender 
and  greatly  elongated;  the  ulna 
is  much  reduced,  only  the  upper 
third  remaining  and  this  is  co-ossified  with  the  radius. 
The  first  digit,  or  thumb,  is  divaricated  from  the 
others,  is  not  included  in  the  wing-membrane  and 
bears  a  curved,  hook-like  claw.  The  bones  of  the 


FIG.  2.  Bones  of  left 
flipper  of  Greenland 
Right  Whale.  Letters 
as  in  Pig.  1.  (From 
Men.) 


48 


THE  THEORY  OF  EVOLUTION 


other  digits  are  extremely  long  and  slender  and 
usually  the  terminal  joints,  with  their  claws,  have 
been  suppressed,  but  in  the  fruit-eating  bats,  or 
"flying  foxes,"  the  second 
digit  retains  the  terminal 
joint  and  claw  and  in  cer- 
tain other  families  the  third 
digit  (but  not  the  sec- 
ond) has  the  terminal  joint, 
though  without  a  claw.  The 
wing  is  a  fold  of  skin,  which 
is  attached  along  the  outer 
side  of  the  body  and  the 
under  side  of  the  arm  and 
forearm  and  is  stretched  be- 
tween the  bones  of  the  digits, 
which  support  it  in  much  the 
same  fashion  as  the  ribs  of 
an  umbrella  sustain  the  cloth. 
The  membrane  is  also  at- 
tached to  the  legs  and  ex- 
tends between  the  two  thighs 
and  the  short  tail. 

In  the  wing  of  a  flying 
bird  the  problem  of  making 
an  organ  of  strong  and  steady  flight  is  solved  in  an 
entirely  different  manner.  The  effective  air-plane 
is  formed  by  large  quill-feathers,  which  are  inserted 
along  the  outer  side  of  the  bones  of  the  arm  and 
hand,  so  there  is  no  wing-membrane  and  the  stiffness 


FIG.  3.  Skeleton  of  right  wing 
of  Leaf-nosed  Bat.  Letters 
as  in  Fig.  1. 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  49 


of  the  feathers  renders  unnecessary  any  stretching 
between  bony  supports.  These  supports  are  there- 
fore developed  according  to  another  plan.  The 
forearm  bones  are  compar- 
atively short  and  strong, 
complete  and  separate  from 
each  other;  the  structure 
of  the  hand  varies  some- 
what in  the  different  groups 
of  birds  and  it  will  there- 
fore be  simpler  to  take  as  a 
type  a  particular  bird,  the 
common  raven.  Here,  the 
bones  of  the  wrist  are  re- 
duced to  two,  partly  by  sup- 
pression and  partly  by  co- 
ossification  of  the  originally 
more  numerous  elements. 
There  are  three  digits,  of 
which  the  first,  or  thumb, 
is  free,  while  the  long  bones 

.    FIG.  4.    Skeleton  of  left  wing  of 
(metacarpals)  OI  the  Second       the   American    Raven.     Let- 

and  third  are  co-ossified;  SjyfaIli>L  (AfterShu' 
the  fingers  are  represented 

by  one  or  two  free  joints  in  each  digit.  In  many 
groups  of  birds  the  first  digit  has  a  claw,  as,  for  ex- 
ample, the  spur  on  a  swan's  wing,  and  several  birds 
have  claws  on  both  the  first  and  second  digits. 

In  the  flightless  birds,  which,  there  is  every  reason 
to  believe,  are  descended  from  flying  ancestors,  the 


50  THE  THEORY  OF  EVOLUTION 

wing-bones  may  be  like  those  of  the  flying  birds  in 
form  and  number,  but  are  altogether  too  small  to 
be  of  any  functional  importance.  This  is  the  case 
in  the  ostrich,  for  example,  while  in  the  cassowary 
the  digits  are  reduced  to  a  single  one,  which  is  pre- 
sumably the  third.  In  the  penguins,  which  are  in- 
capable of  flight,  the  wing  is  not  useless,  but  is 
adapted  to  a  new  purpose  and  has  been  transformed 
into  a  flipper  or  swimming  paddle.  This  change  of 
function  does  not  involve  much  modification  of  the 
wing-bones,  except  that  those  of  the  hand  are  much 
elongated  and  flattened;  the  transformation  is  most 
marked  in  the  feathers.  It  might  almost  be  said 
that  these  most  curious  birds  have  been  adapted  to 
flight  in  a  new  medium,  water  instead  of  air. 

Still  a  third  type  of  wing  is  exemplified  by  the 
remarkable  group  of  extinct  flying  reptiles  known 
as  pterodactyls.  In  these  creatures,  some  of  which 
attained  imffiense  size,  the  wing  was  most  like  that 
of  the  bats,  being  a  membrane  or  fold  of  skin,  naked 
and  without  hair,  scales  or  feathers.  Only  one  digit, 
the  fifth,  or  little  finger,  was  elongated  and  strength- 
ened to  support  the  wing-membrane  along  its  outer 
border,  while  the  other  digits  were  free  and  provided 
with  claws.  In  all  three  groups,  bats,  birds  and 
pterodactyls,  a  vertical  keel  grows  upon  the  breast- 
bone (as  any  one  may  see  in  carving  the  breast  of  a 
fowl),  in  order  to  furnish  a  sufficient  bony  attach- 
ment to  the  powerful  and  greatly  developed  breast- 
muscles,  which  are  the  principal  muscles  of  flight. 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  51 

The  size  and  prominence  of  this  keel  are  excellent 
indications  as  to  the  power  of  prolonged  and  steady 
flight. 

Many  other  examples  might  be  given  in  illustration 
of  the  principle  that,  among  the  Vertebrates,  we  are 
dealing  with  countless  modifications  of  a  single  type, 
with  lower  and  higher  grades  of  organization  and 
adaptation  to  a  great  variety  of  habits  of  life,  ter- 
restrial, burrowing,  climbing,  swimming,  flying,  etc., 
etc.  The  hypothesis  of  evolution  offers  the  simplest 
and  most  natural  explanation  of  the  facts. 

Similar  considerations  apply  to  the  other  great 
types  of  animal  structure.  Among  the  Crustacea, 
for  example,  we  may  take  the  common  lobster, 
which  is  one  of  the  most  advanced  of  these  animals. 
There  is  a  large  anterior  shield,  or  carapace,  which 
is  a  single  piece  and,  behind  this  is  a  number  of 
movable  rings  or  segments,  which  compose  the 
abdomen  or  tail.  To  each  segment  is  attached  a 
pair  of  jointed  appendages,  which  are  the  swimmerets, 
each  of  which  is  divided  into  external  and  internal 
portions.  The  last  pair  of  the  swimmerets  is  very 
much  expanded  and  forms  with  the  terminal  seg- 
ment a  broad  tail-fin.  When  the  lower  surface  of 
the  lobster  is  examined,  it  is  seen  that  the  region 
covered  by  the  carapace  is  likewise  composed  of 
segments,  each  with  its  pair  of  jointed  appendages 
but  in  this  region  the  segments  are  immovably  fixed 
together  and  the  appendages  are  altogether  different 
in  appearance  from  those  of  the  tail.  The  four 


52  THE  THEORY  OF  EVOLUTION 

posterior  pairs  are  walking  legs,  which  correspond 
to  the  internal  division  of  the  swimmerets.  In  front 
of  the  legs  is  the  pair  of  great  claws,  which  are  com- 
posed of  the  same  number  of  joints,  arranged  in  the 
same  way,  only  very  much  larger,  heavier  and  more 
powerful.  In  front  of  the  claws  come  three  pairs 
of  appendages  called  "foot-jaws"  (maxillipeds) 
which  are  legs  with  their  basal  joints  forming  organs 
of  mastication;  the  hindmost  pair  of  the  foot- jaws 
differs  but  little  from  the  walking  legs,  while  in  the 
foremost  pair  it  is  difficult  to  make  out  the  corre- 
sponding parts.  Next  in  order  come  three  pairs  of 
jaws,  still  more  modified,  then  the  very  long  and 
many  jointed  feelers  (antennae)  and  the  anterior 
pair  of  much  shorter  feelers  (antennules) .  The  ap- 
pendages of  the  head,  body  and  tail  are  thus  seen 
to  be  modifications  of  a  common  plan  and  grade  into 
each  other  almost  imperceptibly. 

If  representatives  of  the  many  families  and  orders 
of  the  Crustacea  be  compared,  it  is  seen  that  there 
is  great  variety  in  the  number  of  segments  which 
are  united  to  form  the  head  and  trunk,  as  well  as 
in  the  form  and  function  of  the  appendages.  The 
appendages  of  the  same  segment,  counting  from  the 
anterior  end,  are  in  one  group  a  pair  of  jaws,  in 
another  foot-jaws  and  in  a  third  walking  legs.  The 
comparison  immediately  suggests  the  derivation  of 
all  the  Crustacea  from  ancestors  in  which  all  the 
segments,  except  the  head,  were  similar  and  were 
provided  with  appendages  which  were  similar 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  53 

throughout.  As  this  group  or  that  advanced  in 
differentiation  certain  appendages  became  specialized 
for  the  better  performance  of  particular  operations. 

In  the  crabs  may  be  observed  a  higher  grade  of 
organization  than  in  the  lobsters,  for  the  crabs,  at 
first  sight,  appear  to  have  lost  the  tail  entirely. 
Turning  a  crab  on  its  back,  however,  the  segmented 
tail  is  seen  to  be  folded  under  the  body  and  lying 
in  a  groove  on  the  ventral  surface;  the  appendages 
of  the  tail  have  been  almost  completely  suppressed. 

Even  more  remarkable  is  the  series  of  modifica- 
tions brought  to  light  by  a  comparative  study  of 
the  mouth-parts  among  the  insects,  in  which  far- 
reaching  modifications  have  been  brought  about  in 
adaptation  to  different  kinds  of  food  and  different 
habits  of  life.  Fundamentally,  the  parts  are  the 
same  throughout,  but  the  transformation  is  fre- 
quently so  profound  as  to  obscure  this  fact.  In 
insects,  such  as  the  cockroach,  which  live  upon  solid 
food,  there  are  masticating  jaws;  in  the  bumble  bee 
a  licking  apparatus;  in  the  butterflies  and  moths  a 
long,  tubular  proboscis  for  sucking  the  nectar  from 
flowers;  in  the  mosquito  a  most  elaborate  arrange- 
ment for  piercing  the  skin  and  sucking  the  blood  of 
its  victim.  "But  always  it  is  the  same  individual 
parts,  on  the  same  definite  segments,  which  are  re- 
peated according  to  rule  as  regards  number  and 
form,  but  with  a  different  mode  of  use;  a  pair  of 
upper  jaws,  a  first  and  second  pair  of  lower  jaws,  but 
the  latter  may  be  long  drawn  out  and  rolled  up  in 


54  THE  THEORY  OF  EVOLUTION 

tubular  form,  and  the  former  may  be  transformed 
from  jaws  into  piercing  lancets.  This  arrangement 
according  to  a  definite  rule,  despite  the  differences 
of  form  and  use,  finds  its  most  probable  explanation 
in  community  of  descent."  l 

Examples  of  the  same  kind,  drawn  from  all  the 
great  divisions  of  the  animal  kingdom,  might  be  in- 
definitely multiplied,  but  such  multiplication  would 
serve  no  useful  purpose,  even  were  it  feasible  within 
our  limits  of  time.  The  principle  of  numerous  modi- 
fications of  a  common  plan,  adapted  to  a  great 
variety  of  habits  of  life  is  made  sufficiently  clear  by 
the  few  instances  already  given. 

The  objection  has  been  frequently  urged,  just  as  in 
the  case  of  classification,  that  this  diversification  of 
a  single  type  of  structure  is  no  proof  of  a  genetic 
connection,  or  community  of  descent,  but  that  the 
connection  is  purely  ideal,  the  manifestation  of  a 
creative  plan.  The  answer  is  the  same  in  both 
cases;  acceptance  of  the  theory  of  evolution  by  no 
means  excludes  belief  in  a  creative  plan,  but  that 
theory  offers  the  most  satisfactory  solution  of  the 
problem.  Another  and  perhaps  weightier  objection 
is  that  comparative  anatomy  gives  us  no  means  of 
connecting  animals  of  fundamentally  different  types 
or  plans  of  structure.  It  is  impossible  to  derive  a 
fish  from  a  lobster,  or  a  starfish  from  an  oyster  and 
thus  the  different  structural  types  would  seem  to  be 
separated  by  impassable  barriers.  Evolution  within 

1Otto  Maas,  in  Die  Abstammungslehre,  Jena,  1911,  pp.  258-9. 


EVIDENCE  FROM  COMPARATIVE  ANATOMY  55 

the  type  might  be  admitted,  without  conceding  the 
possibility  of  deriving  one  type  from  another. 

In  reply,  attention  must  be  called  to  the  fact  that 
comparative  anatomy  deals  only  with  the  animal 
world  as  it  now  exists  and  that  is  demonstrably  but 
a  very  small  part  of  the  countless  horde  of  animal 
forms  that  formerly  inhabited  the  earth,  but  have 
become  extinct.  An  oft-used  illustration  of  the  re- 
lations between  living  and  extinct  organisms  is  that 
of  a  great  tree  buried  in  the  earth,  so  that  only  the 
outermost  twigs  are  exposed  to  view.  Such  twigs 
would  form  scattered  groups,  more  or  less  widely 
separated  from  one  another,  the  branches  which  con- 
nect them  being  buried  out  of  sight.  If  it  were 
possible  to  dig  away  the  earth  and  expose  the  whole 
tree,  the  continuity  of  all  its  ramifications  would  be 
fully  revealed.  To  a  certain  extent,  this  excavation 
is  accomplished  by  the  science  of  palaeontology, 
which  deals  with  the  remains  of  extinct  animals  and 
plants  as  preserved  in  the  rocks  of  the  earth's  crust. 
Another  lecture  (No.  IV)  will  be  devoted  to  the  tes- 
timony of  this  science  to  the  evolutionary  hypothesis. 
Another  illustration  may  be  drawn  from  a  study  of 
modern  languages,  as  it  would  be  if  that  study  were 
deprived  of  the  help  given  by  books  and  manuscripts, 
which  register  past  changes,  and  confined  to  an  ex- 
amination of  the  spoken  tongues.  This  examina- 
tion would  suffice  to  make  clear  the  interrelationships 
of  most  European  languages;  French,  Italian,  Por- 
tuguese, Spanish,  etc.,  the  Romance  languages,  are 


56  THE  THEORY  OF  EVOLUTION 

evidently  derived  from  a  common  source.  Another 
group  of  common  origin  is  that  of  the  Teutonic 
languages,  German,  Danish,  Swedish,  Dutch,  Eng- 
lish, etc.,  while  a  third  group,  that  of  the  Slavonic 
tongues,  is  a  natural  assemblage  of  related  languages, 
as  is  that  of  the  Celtic  group.  A  wider  comparison 
indicates  that  all  of  these  European  languages,  to- 
gether with  Persian,  Hindustani,  Lithuanian,  etc., 
form  a  related  group,  the  Indo-European,  in  con- 
tradistinction to  the  so-called  Semitic  and  Turanian 
families.  It  is  an  obvious  conclusion  that  all  of  the 
Indo-European  languages  had  a  common  origin  at  a 
very  ancient  date  and  that  this  common  speech  split 
up  into  a  number  of  branches,  each  of  which,  in 
turn,  became  divided  into  many  languages  and 
dialects. 

The  argument  from  comparative  anatomy  in  fa- 
vour of  the  evolutionary  hypothesis  would  be  notably 
strengthened  by  a  consideration  of  rudimentary 
organs,  but  this  line  of  evidence  can  best  be  dealt 
with  in  connection  with  embryology. 


LECTURE  IH 
EVIDENCE  FROM  EMBRYOLOGY  AND  BLOOD  TESTS 

JEmbryology  .is  the-  study  of  the  development  of 

the  individual  organsim  from  its  beginning  in  the 
egg  to  the  attainment  of  the  adult  condition.  This 
individual  development  is  called  ontq^er^/  and  flip 
question  of  the  relation  of  ontogeny  to  the  ancestral  j- 
history  of  the  species,  or  ?>ht/7sKiftiyt jyj|irtitfftTT?m°:  ' 
of  the  main  problems  of  embryology.  Around  this 
problem  many  controversies  have  raged,  contro- 
versies which  have  by  no  means  arrived  at  a  definite 
solution,  even  to-day.  Thirty  years  ago  the  "re- 
capitulation theory"  was  well-nigh  universally  ac- 
cepted, according  to  which  the  individual  develop- 
ment, or  ontogeny^  was  regarded  as  an  abbreviated— • 
repetition  of  the  ancestral  history  of  the  species,  or 
phylogeny.  Haeckel  called  this  theory  the  "fun- 
damental biogenetic  law"  and  upon  it  he  established 
his  whole  "History  of  Creation."  Nowadays,  that 
"fundamental  law"  is  very  seriously  questioned  and 
by  some  high  authorities  is  altogether  denied.  How- 
ever, even  those  who  take  this  extreme  position  con- 
cerning the  recapitulation  theory  see  in  the  facts  of 
embryology  one  of  the  strongest  supports  of  the 
doctrine  of  evolution. 

57 


58  THE  THEORY  OF  EVOLUTION 

It  was  very  early  recognized  that  the  recapitula- 
tion theory  could  not  be  applied  with  literal  exact- 
ness, but  was  subject  to  certain  important  excep- 
tions and  qualifications. 

_-4!}Jla_Ltlie}  history  must  have  been  enormously 
abbreviated.  After  three  weeks  of  incubation  the 
tiny  speck  of  protoplasm,  which  forms  a  circular 
mark  on  the  yolk  of  a  hen's  egg,  is  developed  into  a 
fully  formed  chick,  ready  for  hatching  and  able  in 
large  degree  to  take  care  of  itself.  On  the  other 
hand,  the  evolution  of  birds  from  their  invertebrate 
ancestors,  through  the  fishes,  amphibians,  and  rep- 
tiles, the  separation  of  the  gallinaceous  stock  from 
other  birds  and  the  differentiation  of  this  particular 
species  were  extremely  slow  processes,  extending 
through  unnumbered  millions  of  years.  Admitting 
recapitulation  to  the  fullest  extent,  it  is  evidently 
a  physical  impossibility  that  it  should  be  a  perfect 
repetition  of  phylogeny;  very  much  of  the  long  story 
must  of  necessity  be  omitted. 

- — fl&^Ehrough  alL_the  stages  of  development  the 
embryo  must  be  rendered  able  to  live  and  grow  and 
thrive  through  adaptation  to  its  surroundings  and 
changes  in  its  environment.  .In_sQEae.j3ji.imalsLjde- 
velopment  takes  place  within  the  body  of  the  mother; 
in  others  the  embryo  is  protected  by  the  hard  egg- 
shell, as  in  birds,  while  the  eggs  of  certain  fishes  and 
many  invertebrates  float  freely  in  the  sea  and  are 
almost  without  protection.  Such  differences  in  en- 
vironment necessitate  differences  in  the  mode  of 


EVIDENCE  FROM  EMBRYOLOGY  59 

development,  while  the  presence  or  absence  of  a 
large  amount  of  inert  food-material,  or  yolk,  exerts 
a  great  influence  in  determining  the  steps  of  on- 
togeny. 

(3)  Many  animals  pass  through  a  larval  stage  of 
development,  in  which  the  immature  young  leads 
an  independent  and  self-sustaining  existence,  during 
which  it  is  very  different  in  appearance  and  structure 
from  its  adult  parents.  Familiar  instances  of  this 
mode  of  development  are  to  be  found  in  the  tadpole, 
which  is  the  larva  of  thelrog,  and  the  caterpillar  the 
larva  of  a  Tnitterfly.  Larvae  are  fully  subject  to 
the  struggle  for  existence  and  must  adapt  themselves 
to  their  environment  and  to  changes  in  that  environ- 
ment, exactly  as  do  adults,  if  they  are  to  survive.  In 
this  way  many  changes  are  introduced  into  the 
ontogeny  which  can  have  no  phylogenetic  signifi- 
cance. It  is  found  in  several  known  instances,  that 
nearly  allied  species,  living  under  different  condi- 
tions, have  quite  different  modes  of  ontogeny,  though 
their  ancestral  history  must  have  been  substantially 
identical.  In  one  and  the  same  species  of  marine 
worms,  for  example,  which  inhabits  both  the  warm 
Mediterranean  and  the  cold  waters  of  the  North 
Sea,  the  larva  of  the  northern  form  is  quite  distinct 
from  that  of  the  southern.  In  attempting  to  in- 
terpret the  meaning  of  embryological  facts,  it  is  thus 
necessary  to  distinguish  sharply  between  j&fiS§.,iga- 
tures  which  are  derived  from  a  long  inheritances 
and  are  therefore  called  \palingenetic,  from  those 


60  THE  THEORY  OF  EVOLUTION 

which  have  been  secondarily  introduced  in  re- 
sponse to  the  changing  needs  of  embryonic  or  larval 
life.  These  secondary  features  are  termed  cen- 
ogenetic. 

"If  we  are  compelled  to  admit  that  cenogenetic 
characters  are  intermingled  with  palingenetic,  then 
we  cannot  regard  ontogeny  as  a  pure  source  of  evi- 
dence regarding  phyletic  relationships.  Ontogeny 
accordingly  becomes  a  field  in  which  an  active  imag- 
ination has  full  scope  for  its  dangerous  play,  but  in 
which  positive  results  are  by  no  means  everywhere 
to  be  obtained.  To  attain  such  results,  the  palin- 
genetic and  cenogenetic  phenomena  must  be  sifted 
apart,  an  operation  which  requires  more  than  one 
critical  grain  of  salt.  On  what  grounds  shall  this 
critique  be  based?  assuredly  not  by  way  of  a  vicious 
circle  on  the  ontogeny  again;  for  if  cenogenetic 
characters  are  present  in  one  case,  who  will  guarantee 
that  a  second  case,  used  for  a  comparison  with  the 
first,  does  not  likewise  appear  in  cenogenetic  dis- 
guise? If  it  once  be  admitted  that  not  everything  in 
development  is  palingenetic,  that  not  every  onto- 
genetic  fact  can  be  accepted  at  its  face  value,  so  to 
speak,  it  follows  that  nothing  in  ontogeny  is  imme- 
diately available  for  the  critique  of  embryonic 
development.  The  necessary  critique  must  be  drawn 
from  another  source."  l 

These  remarks  of  Gegenbaur's  were  called  forth 
by  the  state  of  wild  speculation  into  which  embryo- 

1 C.  Gegenbaur,  Morphologisches  Jahrbuch,  Vol.  XV,  p.  5. 


EVIDENCE  FROM  EMBRYOLOGY  61 

logical  work  had  fallen.  As  there  were  no  generally 
accepted  canons  of  interpretation  for  the  facts  of 
embryological  development,  different  writers  inter- 
preted these  facts  in  the  most  divergent  and  con- 
tradictory manner,  resulting  in  a  chaotic  confusion, 
which  led  to  a  strong  reaction  against  the  whole 
method,  though  there  can  be  little  doubt  that  this 
reaction  has  gone  too  far. 

"It  must  be  evident  to  any  candid  observer,  not 
only  that  the  embryological  method  is  open  to 
criticism,  but  that  the  whole  fabric  of  morphology, 
so  far  as  it  rests  upon  embryological  evidence,  stands 
in  urgent  need  of  reconstruction.  For  twenty  years 
embryological  research  has  been  largely  dominated 
by  the  recapitulation  theory;  and  unquestionably 
this  theory  has  illuminated  many  dark  places  and 
has  solved  many  a  perplexing  problem  that  without 
its  aid  might  have  remained  a  standing  riddle  to  the 
pure  anatomist.  But  while  fully  recognizing  the  real 
and  substantial  fruits  of  that  theory,  we  should  not 
close  our  eyes  to  the  undeniable  fact  that  it,  like 
many  another  fruitful  theory,  has  been  pushed  be- 
yond its  legitimate  limits.  It  is  largely  to  an  over- 
weening confidence  in  the  validity  of  the  embryo- 
logical  evidence  that  we  owe  the  vast  number  of  the 
elaborate  hypothetical  phylogenies  which  confront 
the  modern  student  in  such  bewildering  confusion. 
The  inquiries  of  such  a  student  regarding  the  origin 
of  any  of  the  great  principal  types  of  animals  involve 
him  in  a  labyrinth  of  speculation  and  hypothesis  in 


62  THE  THEORY  OF  EVOLUTION 

which  he  seeks  in  vain  for  conclusions  of  even  an 
approximate  certainty."  1 

Many  other  equally  vigorous  and  well-deserved 
criticisms  of  the  embryological  method  might  be 
cited,  but  it  should  be  emphasized  that  these  crit- 
icisms are  all  directed  against  the  application  of  the 
method  to  the  solution  of  definite  and  concrete 
problems  of  descent  and  relationship.  None  of  them 
denies  and  many  strongly  affirm  that  embryology 
affords  some  of  the  strongest  and  most  convincing 
evidence  in  favor  of  the  evolutionary  theory. 

Let  us  examine  some  of  this  evidence.  To  begin 
with,  it  should  be  noted  that,  in  following  out  the 
ontogeny  or  individual  development,  the  observer 
witnesses  the  formation  of  something  new,  not 
merely  the  enlargement  and  unfolding  of  a  pre- 
existing organism,  though  the  theory  of  preformationjjLLf 
which  was  widely  accepted  in  the  eighteenth  century, 
looked  upon  ontogeny  precisely  in  that  way,  as  the 
growth  of  a  germ  which  was  the  miniature  of  the 
parent.  Such  a  theory  was  possible  only  before  the 
development  of  microscopic  technique  had  enabled 
the  observer  to  detect  the  actual  successive  steps  of 
change.  The  egg  is  a  single  cell,  with  the  nucleus  and 
all  the  parts  of  other  undifferentiated  cells,  though  it 
may  be  enormously  enlarged  by  the  presence  of  food- 
yolk,  jn  the  hen's  egg  this  food-yolk  is  quite  inert 
and  the  activity  of  development  isconfined  to  the 
minute  disc  of  protoplasm  on  the  outside  of  the  yolk, 

1 E.  B.  Wilson,  in  Wood's  Hole  Biological  Lectures,  1894,  p.  103. 


EVIDENCE  FROM  EMBRYOLOGY 


63 


while  i  i  the  frog's  egg  the  yolk  is  disseminated, 
though  not  uniformly,  throughout  the  egg  and  in  the 
mammalian  egg,  which  is  microscopic  in  size,  there  is 


FIG.  5.     Embryos  in  corresponding  stage  of  development  of 
Shark  (A),  Fowl  (B),  and  Man  (C).  g,  gill-slits. 

no  yolk.  It  is  a  very  remarkable  fact  that  all  of  the 
vertebrated  animals,  fishes,  amphibians,  reptiles, 
birds  and  mammals,  however  different  their  habits 
and  modes  of  life,  have  a  mode  of  ontogeny  which  is 
of  even  more  characteristically  and  unmistakably 
the  same  plan  than  is  the  type  of  their  adult  struc- 


64  THE  THEORY  OF  EVOLUTION 

ture,  which  was  described  in  the  last  lecture^  _  The_ 
egg,  or  the  active  portion  of  it.  divides  in  a  definite 
and  regular  manner  into  a  very  large  number  of  cells, 
which  arrange  themselves  in  definite  layers,  an  outer 
and  an  inner,  and  within  these  layers  cell-aggregates 
form  incipient  organs,  which,  step  by  step,  take  on 
the  adult  condition.  Not  only  is  the  plan  and  type  of 
development  essentially  similar  throughout  the  whole 
phylum  of  the  vertebrates,  but,  in  accordance  with 
the  recapitulation  theory,  many  structural  features 
which  are  permanent  in  lower  forms  appear  in  the 
embryos  of  higher  and  more  advanced  types.  In 
the  latter,  however,  these  features  are  transitory  and, 
in  the  course  of  development,  they  either  disappear, 
or  are  so  modified  as  to  be  very  different,  sometimes 
unrecognizable,  in  the  adulj:^.  ^  uIVT..tv 

j\i  a  pprtain  stagp  of  the  pnfogftny  tTip  pmhryo  of  a 

mammal  has  gill-pouches  like  a  fish,  the  skeletal 
supports  of  the  gill-pouches,  the  arteries  and  veins 
which  supply  them  with  blood,  the  structure  of  the 
heart,  in  short,  the  entire  plan  of  the  circulatory 
system  is  fish-like.  At  a  later  stage  most  of  the  gillz 
pouches  have  been  obliterated,  but  one  is  retained 
and  converted  into  the  Eustachian^anal.  which 
connects  the  throat  with  the  middle  ear,  inside  of  the 
ear-drum.  Similarly,  the  embryological  evidence 
shows  that  the 


^_ 

derived  from  the  swim-bladder  of  fishes,  a  conclusion 
which  had  already  been  reached  by  comparative 
anatomy,  for  in  a  remarkable  group,  known  as  the 


EVIDENCE  FROM  EMBRYOLOGY  65 

Dipnoi  or  lung-fishes,  the  air-bladder  is  utilized  for 
purposes  of  respiration. 

It  has  been  objected  that,  while  embryology  may 
prove  relationship  within  a  single  type,  it  fails  to 
demonstrate  any  connection  between  different  types, 
but  this  is  not  altogether  true.  The  Tunicata,  a 
curious  group  of  marine  animals  once  referred  to  the 
Mollusca,  are  shown  by  their  ontogeny  to  be  related 
to  the  vertebrates  and  the  same  is  true  of  certain 
marine  worms  (Balanoglossus) .  Indeed,  most  mod- 
ern zoologists  have  adopted  a  scheme  of  classifica- 
tion, in  which  the  type  Chordata  includes  not  only 
the  true  vertebrates,  but  also  the  Lancelet  (Am- 
phioxus),  the  tunicates  and  Balanoglossus;  this 
scheme  is  founded  upon  the  embryological  evidence. 
Among  the  invertebrates  even  more  remarkable  ex- 
amples have  been  observed.  Such  radically  differ- 
ent types  as  the  segmented  worms  and  the  shell-fish 
(Mollusca)  are  brought  into  relationship  by  their 
ontogeny  and  their  closely  similar  types  of  larvae,  as 
are  also,  though  less  distinctly,  the  brachiopods  or 
lamp-shells,  and  the  Bryozoa.  The  Horseshoe-crab, 
or  King-crab,  so  abundant  along  our  Atlantic  coast, 
was  long  of  uncertain  affinities;  originally  referred  to 
the  Crustacea,  largely  because  of  its  marine  habits 
of  life,  embryology  makes  much  more  probable 
its  relationship  to  the  air-breathing  scorpions  and 
spiders,  a  result  which  will  be  examined  again  from 
another  point  of  view  in  connection  with  blood-tests. 

Even  before  the  publication  of  Darwin's  "Origin  of 


66  THE  THEORY  OF  EVOLUTION 

Species"  one  of  the  great  stumbling  blocks  in  the 
way  of  the  theory  of  special  creation  was  the  existence 
in  a  great  many  animals  of  rudimentary  organs,  or 
such  as  are  so  far  reduced  and  atrophied  as  to  be  of 
no  service  to  their  possessors.  An  analogy  employed 
by  my  lamented  friend,  Mr.  Richard  Lydekker,  may 
be  advantageously  repeated  here.  Let  us  suppose 
that  a  screw-steamer,  with  longitudinal  shaft  lead- 
ing aft  from  the  engine-room  to  the  stern,  where  it 
carries  the  propeller,  should,  on  close  examination, 
reveal  many  signs  that  it  had  originally  been  a  "side 
wheeler,"  or  paddle-boat.  Recognizable  remnants  of 
paddle-boxes,  of  bearings  for  a  transverse  shaft,  and 
the  like,  are  found;  what  would  be  the  inevitable 
conclusion?  No  one  would  maintain  that  a  naval 
architect,  in  possession  of  his  senses,  in  constructing  a 
screw-steamer  would  deliberately  introduce  features 
which  are  useful  and  appropriate  only  in  a  paddle- 
boat.  The  only  reasonable  explanation  would  be 
that  the  vessel  had  originally  been  built  as  a  paddle- 
boat  and  had  subsequently  been  converted  into  a 
screw-steamer  and  in  the  conversion  it  had  not  been 
found  necessary  completely  to  eradicate  all  traces  of 
the  original  construction.  Obviously,  the  same  rea- 
soning applies  to  rudimentary  organs.  The  only 
satisfactory  explanation  of  such  useless  remnants  is 
that  their  possessors  are  descendants  of  ancestors  in 
which  those  organs  were  fully  functional.  It  seems 
quite  absurd  to  assume  that,  in  a  separately  and 
specially  created  animal,  useless  structures,  reminis- 


EVIDENCE  FROM  EMBRYOLOGY  67 

cent  of  other  animals  in  which  the  same  structures 
are  useful  and  valuable,  should  be  included,  merely 
to  indicate  ideal  relationships  and  community  of 
plan. 

It  was  sought  to  break  the  force  of  this  very  serious 
objection  to  the  theory  of  special  creation  by  saying 
that  apparently  useless  organs  may  nevertheless 
have-  functions  which  are  still  unknown  to  us  and 
may  be  revealed  by  future  discovery.  In  certain 
cases,  like  that  of  the  thyroid  gland  in  the  neck,  this 
contention  has  been  justified,  but  there  are  many 
others  to  which  it  does  not  apply.  For  example, 
in  the  great  and  varied  whale-tribe  (order  Cetacea) 
which  includes  the  right,  or  whalebone,  whales,  the 
sperm-whales,  the  porpoises,  dolphins,  etc.,  the  fore- 
limbs  have  been  converted  into  swimming  paddles, 
but  the  hind  limbs  appear  to  have  vanished  com- 
pletely, leaving  no  externally  visible  trace.  In- 
ternally, however,  recognizable  remnants  of  the  hind 
limb-bones  may  be  found  in  various  stages  of  reduc- 
tion, which  differ  in  the  different  members  of  the 
order.  In  the  Greenland  Right  Whale  the  hip-bone, 
thigh-bone  and  shin-bone  are  indicated;  in  the  Fin- 
whale  only  the  hip-bones  and  a  minute  rudiment  of 
the  thigh-bone  are  to  be  found;  in  the  toothed  whales 
only  an  almost  unrecognizable  remnant  of  the  hip- 
bone is  left  and  in  one  of  the  dolphins  even  that  has 
disappeared.  Similarly,  the  snakes  have  lost  their 
limbs  completely,  so  far  as  external  appearance  is 
concerned,  and  in  most  members  of  the  group  no 


68  THE  THEORY  OF  EVOLUTION 

trace  of  limbs  is  to  be  found  on  dissection,  but  in 
certain  snakes  the  rudiments  of  limbs  are  to  be 
detected.  Leaving  aside  all  preconceptions,  which 
is  the  more  probable  explanation  of  such  phenomena, 
the  theory  of  special  creation  or  the  theory  of  evolu- 
tion? 

Even  if  it  were  admitted  that  all  rudimentary 
organs  and  structures  found  in  the  adult  have  a 
certain  unknown  use  and  value,  no  one  could  main- 
tain this  with  regard  to  the  countless  instances  of 
structures  which  are  developed  in  the  embryo,  but 
disappear  entirely  before  birth.  It  is  possible  to 
mention  but  a  very  few  of  such  instances  out  of  the 
great  number  that  have  already  been  observed  and  re- 
corded, but  these  few  will  suffice  to  illustrate  the 
principle  involved. 

^Ryamptefr-trf-  this  may  be  cited  from  the  most 
widely  different  gmnpg-  in  tl^  ^mhryo  Of  insects^ 
especially  of  beetles,  pairs  of  legs  are  formed  within 
the_egg,  not  only  on  the  head  and  thorax,  but  also 
on  the  abdomen,  but  while  those  on  the  head  are 
transformed  into  mouth-parts,  those  on  the  thorax 
are  farther  developed  in  their  jointing  and  muscula- 
ture to  be  locomotive  legs,  those  on  the  abdomen 
are  again  resorbed.  In  many  fresh-water  worms,  the 
eggs  of  which  are  laid  in  a  cocoon,  from  which  they 
are  hatched  as  a  finished,  minute,  crawling  worm, 
larval  organs  are  nevertheless  formed,  which  recall 
those  of  the  Trochophore,  the  larva  of  the  original 
worms,  which  swims  freely  in  the  sea.  However, 


EVIDENCE  FROM  EMBRYOLOGY  69 

these  larval  organs  .  .  .  are  never  properly  func- 
tional, since  no  actually  free-swimming  larva  is 
developed,  but  the  embryo  merely  floats  in  the  al- 
buminous fluid  of  the  cocoon. 

"A  particularly  beautiful  example  is  offered  by 
the  whales  in  their  embryological  development,  which 
has  been  thoroughly  studied  by  Kiikenthal.  In  the 
adult  condition  they  show  only  the  anterior  ex- 
tremities, but  in  the  embryo  the  posterior  pair,  with 
their  skeletal  parts,  are  formed,  but  are  afterwards 
completely  atrophied.  Although  they  are  mammals, 
in  the  adult  condition  they  have  absolutely  no 
covering  of  hair,  since  in  their  aquatic  life  another 
and  more  effective  pfotection  against  loss  of  heat  is 
given  by  means  of  a  thick  layer  of  blubber;  only  a 
few  coarse  bristles,  partly  with  particular  functions, 
have  persisted  on  a  few  parts  of  the, body.  But  in 
the  embryo  a  dense  covering  of  hair  is  formed,  which 
is  later  transformed  in  a  peculiar  manner  and 
atrophied.  Further,  a  series  of  whales  have  no 
_te^th_in_jiie_jELdult_^ondition,  but  only  the  well- 
known,  eel-trap  like,  horny  plates,  from  which  whale- 
bone is  produced.  Nevertheless,  in  the  embryo  there 
is  a  dentition  of  numerous  teeth,  which  are,  however, 
resorbed,  without  ever  piercing  the  gum."  1 

Throughout  the  great  group  of  the  ruminants, 
which  includes  the  oxen,  buffaloes,  bison,  sheep, 
goats,  antelopes,  deer  and  giraffes,  the  collar-bone  is 
invariably  lacking,  since  it  is  superfluous  on  account 

1  Otto  Maas,  in  Die  Abstammungslehre,  pp.  273-4. 


70  THE  THEORY  OF  EVOLUTION 

of  the  exclusively  locomotive  manner  in  which  the 
fore  legs  are  employed.  Tn  the  embryo  sheep J:he 
collar-bone  is  established  and  even,  to  some  extent, 
ossified,  but  is  subsequently  resorbed  and  disappears 
entirely.  No  doubt,  the  collar-bone  will  be  found 
in  many  other  embryo  ruminants,  when  the  proper 
examination  shall  have  been  made,  but  its  demon- 
strated presence  in  the  foetal  sheep  is  sufficiently 
striking.  In  the  higher  mammals  the  number  of 
teeth  was  originally  44,  or  11  on  each  side  of  both 
upper  and  lower  jaws,  but  in  most  of  the  modern  or 
existing  groups  of  these  higher  mammals  this  number 
has  been  very  considerably  reduced  through  the 
suppression  of  certain  teeth.  We  have  every  reason 
to  believe  that  the  ancestors  of  the  forms  with  re- 
duced dentition  possessed  teeth  in  full  numbers  and 
that  there  has  actually  been  a  loss  of  teeth  in  the 
course  of  descent.  This  conclusion  is  abundantly 
confirmed  by  the  facts  of  embryology.  Take,  for 
example,  the  great  group  of  the  gnawing  mammals, 
or  Rodentia,  in  which  the  front  teeth  or  incisors, 
above  and  below  are  reduced  to  one  on  each  side, 
except  in  the  rabbits.  The  incisors  are  chisel-shaped 
and  are  faced  with  hard  enamel,  so  that  the  action 
of  the  upper  teeth  upon  the  lower  keeps  the  cutting 
edges  extremely  sharp;  these  teeth  do  not  form 
roots,  but  continue  to  grow  throughout  the  lifetime 
of  the  animal.  Between  the  chisel-like  incisors  and 
the  grinding  teeth,  there  is  a  long,  toothless  gap, 
which,  we  assume,  was,  in  the  ancestors  of  the 


EVIDENCE  FROM  EMBRYOLOGY  71 

rodents,  occupied  by  the  second  and  third  incisors, 
the  canine  and  two  or  more  grinders.  This  conclu- 
sion is  justified  by  the  facts  of  embryology;  for  in- 
stance, hi  the  embryo  of  the  squirrelseveral  of  the 
-jnjssing  teeth  are  be^un  as  distinct  tlwth-ffpnTTfi  hl1T" 

and  are  resorbed 


j>efore  birth. 

All  available  evidence  points  to  the  conclusion 
that  birds  are  descended  from  reptiles,  a  conclusion 
which  is  especially  strengthened  by  the  facts  of 
palaeontology  and  will  be  examined  more  at  length 
in  the  following  lecture.  Such  a  descent  explains 
many  otherwise  puzzling  features  in  the  ontogeny 
of  birds,  in  which  reptilian  characteristics  appear  in 
transitory  fashion  and  are  either  modified  so  as  to 
take  on  typically  bird-like  character,  or  are  sup- 
pressed altogether.  A  remarkable  example  of  this 
is  the  formation  of  rudimentary  teeth  in  certain 
embryonic  birds,  followed  by  their  resorption  and 
disappearance  before  hatching. 
^J[tcanhardly_be  contended  that  these  rudimentary 
structures,  which  are  confined  to  the  embryonic 
stages  of  development  and  of  which  no  trace  remains 
in  the  adult,  are  so  indispensable  to  the  processes 
of  ontogeny,  that  they  were  specially  created  to 
serve  this  temporary  purpose.  For  such  a  conten- 
tion there  is  not  a  particle  of  evidence  and  the 
theory  of  evolution,  which  regards  these  structures 
as  useless  remnants,  due  to  inheritance  from  an- 
cestors in  which  the  structures  were  functional, 


72  THE  THEORY  OF  EVOLUTION 

offers  much  the  most  satisfactory  solution  of  the 
problem  that  has  yet  been  suggested. 

Embryology  further  shows  that  evolution  is  not 
invariably  an  advance  from  lower  and  simpler  to 
higher  and  more  complex  types,  but  may  be  by  way 
of  degeneration,  and  degradation.  The  adoption  of 
a  parasitic  mode  of  life  is  very  apt  to  cause  such 
degradation,  and  some  very  remarkable  instances  of 
the  degeneration  of  parasites  have  been  observed. 
An  instructive  example  that  may  be  cited  is  that  of 
Sacculina,  a  nondescript  creature  that  is  parasitic 
on  certain  species  of  crabs.  The  parasite  is  attached 
to  the  body  of  its  victim,  underneatTT~the  tail,  by 
means  of  root-like  fibres  which  penetrate  and  ramify 
throughout  the  interior  of  the  crab.  The  root-like 
fibres  absorb  nutriment  and  convey  it  to  the  body 
of  the  parasite,  which  is  reduced  to  a  mere  bag, 
without  appendages,  muscles,  nervous  system,  sen- 
sory apparatus,  digestive  tract,  or  any  determinable 
organs  save  those  of  reproduction.  The  creature 
has  the  power  of  assimilating  the  nutritive  juices 
which  are  conveyed  to  it  by  the  root-like  filaments 
from  the  body  of  its  host,  and  the  power  of  reproduc- 
tion, and  it  must  have  some  respiratory  and  excretory 
capacity,  though  there  are  neither  gills  nor  glands. 
From  an  examination  of  the  adult  parasite  alone, 
it  would  be  quite  impossible  to  classify  it  and  deter- 
mine the  type  and  class  to  which  it  should  be  re- 
ferred, but  embryology  solves  the  problem.  From 
the  egg  is  hatched  a  free-swimming  larva,  which 


EVIDENCE  FROM  EMBRYOLOGY  73 

has  jointed  appendages,  nervous,  muscular  and 
digestive  systems  and,  in  short,  clearly  belongs  to 
that  group  of  the  Crustacea  which  includes  the 
barnacles.  This  is  degeneration  carried  nearly  to 
the  utmost  possible  extreme  and  yet  the  individual 
development  shows  the  derivation  of  this  otherwise 
problematical  parasite  and  the  steps  through  which 
it  passed  in  its  deterioration. 

It  was  stated  above  that  several  distinguished  nat- 
uralists altogether  reject  the  recapitulation  theory  as 
a  means  of  interpreting  the  facts  of  embryology. 
They  do  this  on  the  ground  that,  inasmuch  as  changes 
and  innovations  in  form  or  structure  must  arise  in  the 
germ-plasm,  at  the  very  beginning  of  ontogeny, 
there  is  no  reason  why  such  changes  might  not  in- 
volve the  whole  course  of  embryological  develop- 
ment. To  my  mind  this  a  priori  objection  to  the 
recapitulation  theory  is  quite  without  force  in  view 
of  the  great  body  of  observed  facts,  but  there  is  no 
time  to  enter  upon  a  discussion  of  such  an  abstruse 
and  difficult  problem.  For  our  present  purpose, 
however,  it  is  important  to  note  that  these  objectors 
are  staunch  evolutionists  and  find  in  the  community 
of  mode  in  ontogeny  between  different  classes  of 
organisms  one  of  the  strongest  arguments  in  support 
of  the  evolutionary  doctrine. 

Here  may  be  conveniently  considered  the  very  in- 
teresting and  significant  blood  tests  which  have 
been  made  in  the  last  fifteen  years  by  various  physi- 
ologists and  especially  by  Dr.  George  H.  F.  Nuttall, 


74  THE  THEORY  OF  EVOLUTION 

of  the  University  of  Cambridge.  Though  there  are 
several  methods  of  making  these  tests,  the  "precipi- 
tation method"  employed  by  Dr.  Nuttall  will  be 
quite  sufficient  for  the  ends  sought  in  these  lectures. 
The  method  and  significance  of  the  tests  can  best  be 
explained  by  taking  as  an  example  human  blood, 
which,  of  course,  has  been  most  extensively  and 
minutely  studied,  because  of  its  legal  importance  as 
well  as  its  scientific  interest.  Ordinary  chemical 
analysis  is  unable  to  determine  the  differences  in 
blood-composition  between  various  animals,  but 
that  there  were  important  differences  had  long  been 
understood.  This  was  shown  by  the  fact  that,  in 
performing  the  operation  for  the  transfusion  of  blood, 
it  was  not  practicable  to  substitute  animal  for  hu- 
man blood,  since  the  former  might  cause  serious 
injury  to  the  patient. 

The  precipitation  method  of  making  blood  tests 
is  as  follows:  Freshly  drawn  human  blood  is  allowed 
to  coagulate  or  clot,  which  it  will  do  in  a  few  min- 
utes, if  left  standing  in  a  dish,  and  then  the  serum 
is  drained  away  from  the  clot.  Blood-serum  is  the 
watery,  almost  colourless  part  of  the  blood,  which 
remains  after  coagulation.  Small  quantities  of  this 
serum  are  injected,  at  intervals  of  one  or  two  days, 
into  the  veins  of  a  rabbit  and  cause  the  formation 
in  the  rabbit's  blood  of  an  anti-body,  analogous  to 
the  anti-toxin  which  is  produced  in  the  blood  of  a 
horse  by  the  injection  of  diphtheria  virus.  After 
the  last  injection  the  rabbit  is  allowed  to  live  for 


EVIDENCE  FROM  BLOOD  TESTS  75 

several  days  and  is  then  killed  and  bled,  the  blood 
is  left  until  it  clots  and  the  serum  drained  off  and 
preserved.  The  serum  obtained  thus  from  a  rabbit 
is  called  "anti-human"  serum  and  is  an  exceedingly 
delicate  test  for  human  blood,  not  only  when  the 
latter  is  fresh,  but  also  when  it  is  in  the  form  of  old 
and  dried  blood-stains,  or  even  when  the  blood  is 
putrid.  Stains,  for  example,  are  soaked  in  a  very 
weak  solution  of  common  salt  and,  if  necessary,  the 
blood  solution  is  filtered  until  it  is  quite  limpid  and 
clear.  Into  the  blood  solution  a  few  drops  of  the 
anti-human  serum  are  conveyed  and,  if  the  stains 
are  of  human  blood,  a  white  precipitate  is  formed 
and  thrown  down,  but  if  the  stains  are  of  the  blood 
of  some  domestic  animal,  such  as  a  pig,  sheep,  or 
fowl,  no  such  reaction  follows.  In  the  same  manner 
as  above  described,  we  may  prepare  anti-pig,  anti- 
horse,  anti-fowl,  etc.,  etc.,  sera  by  injecting  the 
fresh-drawn  serum  of  a  pig,  horse,  fowl,  or  any  other 
animal  into  the  rabbit,  instead  of  human  blood- 
serum.  In  some  countries,  notably  in  Germany  and 
Austria,  this  test  has  already  been  adopted  by  the 
courts  of  justice  and  has  been  found  extremely  useful 
in  the  detection  of  crime. 

Further  investigation  showed  that  these  blood 
tests  might  be  employed  to  determine  the  degrees 
of  relationship  between  different  animals,  for,  al- 
though a  prompt  and  strong  reaction  is  usually 
obtained  only  from  the  blood  of  the  same  species  as 
that  from  which  the  original  injection  into  the  rabbit 


76  THE  THEORY  OF  EVOLUTION 

was  taken,  the  blood  of  nearly  allied  species,  such  as 
the  horse  and  donkey,  for  example,  gives  a  weaker 
and  slower  precipitation.  By  using  stronger  solu- 
tions and  allowing  more  time,  quite  distant  relation- 
ships may  be  brought  out.  Nuttall  and  his  collabo- 
rator, Graham-Smith,  made  many  thousands  of  such 
experiments  bearing  upon  the  problems  of  relation- 
ship and  classification  and  it  is  of  great  significance 
to  note  that  their  highly  interesting  and  important 
results  contain  few  surprises,  but,  in  almost  all  cases, 
merely  serve  to  confirm  the  conclusions  previously 
reached  by  other  methods,  such  as  comparative 
anatomy  and  palaeontology.  It  will  be  instructive 
to  quote  some  of  these  results,  the  quotations  being 
taken  from  "Blood  Immunity  and  Blood  Relation- 
ship by  G.  H.  F.  Nuttall,  including  Original  Re- 
searches by  G.  L.  Graham-Smith  and  T.  S.  P. 
Strangeways;"  Cambridge,  1904. 

"In  the  absence  of  palseontological  evidence  the 
question  of  the  interrelationship  amongst  animals 
is  based  upon  similarities  of  structure  in  existing 
forms.  In  judging  of  these  similarities,  the  subjec- 
tive element  may  largely  enter"  (p.  1).  "The  very 
interesting  observations  upon  the  eye  made  by 
Johnson  also  demonstrate  the  close  relationships 
between  the  Old  World  forms  and  man,  the  macula 
lutea  tending  to  disappear  as  we  descend  in  the  scale 
of  New  World  monkeys  and  being  absent  in  the 
Lemurs.  The  results  which  I  published  upon  my 
tests  with  precipitins  directly  supported  this  evi- 


EVIDENCE  FROM  BLOOD  TESTS  77 

dence,  for  the  reactions  obtained  with  the  bloods  of 
Simiidse  [i.  e.,  Man-like  Apes]  closely  resemble  those 
obtained  with  human  blood,  the  bloods  of  Cercopi- 
thecidse  [Old  World  Monkeys]  came  next,  followed 
by  those  of  Cebidae  and  Hapalidee  [New  World 
Monkeys  and  Marmosets]  which  gave  but  slight 
reactions  with  anti-human  serum,  whilst  the  blood 
of  Lemuroidea  gave  no  indication  of  blood-relation- 
ship." (p.  2).  "A  perusal  of  the  pages  relating  to 
the  tests  made  upon  the  many  bloods  I  have  exam- 
ined by  means  of  precipitating  anti-sera,  will  very 
clearly  show  that  this  method  of  investigation  per- 
mits of  our  drawing  certain  definite  conclusions.  It 
is  a  remarkable  fact  .  .  .  that  a  common  property 
has  persisted  in  the  bloods  of  certain  groups  of  ani- 
mals throughout  the  ages  which  have  elapsed  during 
their  evolution  from  a  common  ancestor,  and  this  in 
spite  of  differences  of  food  and  habits  of  life.  The 
persistence  of  the  chemical  blood-relationship  be- 
tween the  various  groups  of  animals  serves  to  carry 
us  back  into  geological  times,  and  I  believe  we  have 
but  begun  the  work  along  these  lines,  and  that  it  will 
lead  to  valuable  results  in  the  study  of  various  prob- 
lems of  evolution"  (pp.  2-^4). 

The  general  conclusions  on  interrelationships,  so 
far  as  they  are  of  particular  interest  for  our  purpose, 
reached  by  Nuttall  and  Graham-Smith  as  the  result 
of  many  thousands  of  blood  tests,  may  be  sum- 
marized as  follows: 

(1)  If  sufficiently  strong  solutions  be  used  and 


78  THE  THEORY  OF  EVOLUTION 

time  enough  be  allowed,  a  relationship  between  the 
bloods  of  all  mammals  is  made  evident. 

(2)  The    degrees    of    relationship    between    man, 
apes  and  monkeys  have  already  been  noted. 

(3)  Anti-carnivore  sera  show   "a  preponderance 
of  large  reactions  amongst  the  bloods  of  Carnivora, 
as  distinguished  from  other  Mammalia;  the  maxi- 
mum reactions  usually  take  place  amongst  the  more 
closely   related   forms  in   the   sense   of  descriptive 
zoology." 

(4)  Anti-pig  serum  gives  maximum  reactions  only 
with  the  bloods  of  other  species  of  the  same  family, 
moderate   reactions   with   those  of   ruminants   and 
camels,  and  moderate  or  slight  reactions  with  those 
of  whales.     Anti-llama  serum  gives  a  moderate  reac- 
tion with  the  blood  of  the  camel,  and  the  close  rela- 
tionship between  the  deer  family  and  the  great  host 
of  antelopes,  sheep,  goats  and  oxen  is  clearly  demon- 
strated. 

(5)  Anti-whale  serum  gives  maximum  reactions 
only  with  the  bloods  of  other  whales  and  slight  reac- 
tions with  those  of  pigs  and  ruminants. 

(6)  A  close  relationship  is  shown  to  exist  between 
all  Marsupials,  with  the  exception  of  the  Thylacine, 
or  so-called  Tasmanian  Wolf. 

(7)  Strong  anti-turtle  serum  gives  maximum  reac- 
tions only  with  the  bloods  of  turtles  and  crocodiles, 
with  those  of  lizards  and  snakes  the  results  are  almost 
negative.     With  the  egg-albumins  of  reptiles  and 
birds  a  moderate  reaction  is  given. 


EVIDENCE  FROM  BLOOD  TESTS  79 

(8)  Anti-lizard  serum  produces  maximum  results 
with  the  bloods  of  lizards  and  reacts  well  with  those 
of  snakes. 

(9)  These  experiments  indicate  that  there  is  a 
close  relationship  between  lizards  and  snakes,  on  the 
one  hand,  turtles  and  crocodiles,  on  the  other.     They 
further  indicate  that  birds  are  more  nearly  allied 
with  the  turtle-crocodile  series  than  with  the  lizard- 
snake  series,  results  for  which  palseontogical  studies 
had  already  prepared  us. 

(10)  "Tests  were  made  by  means  of  anti-sera  for 
the  fowl  and  ostrich  upon  792  and  649  bloods  respec- 
tively.    They  demonstrate  a  similarity  in  blood  con- 
stitution of  all  birds,  which  was  in  sharp  contrast  to 
what  had  been  observed  with  mammalian  bloods, 
when  acted  upon  by  anti-mammalian  sera.     Differ- 
ences in  the  degree  of  reaction  were  observed,  but 
did  not  permit  of  drawing  any  conclusions." 

(11)  I  have  already  called  attention  to  the  fact 
that  the  problematical  Horseshoe-crab  is  indicated 
by  its  embryology  to  be  related  to  the  air-breathing 
spiders  and  scorpions  rather  than  to  the  marine 
Crustacea.     It  is  of  exceptional  interest  to  learn 
that  embryology  is  supported  by  the  results  of  the 
blood-tests. 

It  must  not  be  supposed  that  there  is  any  exact 
mathematical  ratio  between  the  degrees  of  relation- 
ship indicated  by  the  blood  tests  and  those  which 
are  shown  by  anatomical  and  palaeontological  evi- 
dence. Any  supposition  of  the  kind  would  be  imme- 


80  THE  THEORY  OF  EVOLUTION 

diately  negatived  by  the  contrast  between  the  blood 
of  mammals  and  that  of  birds.  It  could  hardly  be 
maintained  that  an  ostrich  and  a  parrot  are  more 
nearly  allied  than  a  wolf  and  a  hyena  and  yet  that 
would  be  the  inference  from  the  blood  tests.  Like 
all  other  anatomical  and  physiological  characters, 
the  chemical  composition  of  the  blood  is  subject  to 
change  in  the  course  of  evolution  and  these  develop- 
mental changes  do  not  keep  equal  pace  in  all  parts 
of  the  organism.  It  is  the  rule  rather  than  the  ex- 
ception to  find  that  one  part  of  the  structure  ad- 
vances much  more  rapidly  than  other  parts,  such  as 
the  teeth,  the  skull,  or  the  feet.  The  human  body 
is,  fortunately  for  us,  of  rather  a  primitive  kind, 
while  the  development  of  the  brain  is  far  superior 
to  that  of  any  other  mammal  and  this  great  brain- 
development  has  necessitated  a  remodeling  of  the 
skull.  On  the  other  hand,  the  skeleton,  limbs, 
hands  and  feet  are  but  slightly  specialized.  In 
the  elephant  tribe,  so  far  as  we  can  trace  them 
back  in  time,  there  has  been  little  change,  save  in 
size,  in  the  structure  of  the  body  or  limbs,  while  the 
teeth  and  skull  have  passed  through  a  series  of  re- 
markable changes.  It  is  for  this  reason  that  it  is 
unsafe  to  found  a  scheme  of  classification,  which  is 
meant  to  be  a  brief  expression  of  relationship,  upon 
a  single  character,  for  the  result  is  almost  invariably 
misleading.  The  results  of  blood  tests  must  be 
critically  examined  and  checked  by  a  comparison 
with  the  results  obtained  by  other  methods  of  inves- 


EVIDENCE  FROM  BLOOD  TESTS  81 

tigation,  but  after  every  allowance  has  been  made, 
these  tests  are  very  remarkable. 

The  blood  tests  have  brought  very  strong  con- 
firmation to  the  theory  of  evolution  and  from  an 
entirely  unexpected  quarter;  they  come  as  near  to 
giving  a  definite  demonstration  of  the  theory  as  we 
are  likely  to  find,  until  experimental  zoology  and 
botany  shall  have  been  improved  and  perfected  far 
beyond  their  present  state. 


LECTURE  IV 
EVIDENCE  FROM  PALAEONTOLOGY 

Palaeontology  is  the  science  which  deals  with  the 
fossils  of  animals  and  plants  entombed  in  the  rocks 
of  the  earth's  crust.  This  science  has  certain  pre- 
eminent advantages  for  the  investigation  of  evolu- 
tionary problems,  because  it  has  to  do  with  the 
recognizable  remains  of  the  actual  organisms  which 
formerly  inhabited  the  surface  of  the  earth  and  which 
are  themselves  the  links  in  the  chain  of  development. 
If  there  be  any  truth  in  the  theory  of  evolution, 
palaeontology  ought  to  be  one  of  its  strongest  supports 
and  offer  some  of  the  most  convincing  testimony  in 
its  favour,  and,  on  the  other  hand,  if  it  be  false,  the 
fossils  should  be  able  to  expose  its  fallacies  in  an  over- 
whelming manner. 

Here,  again,  it  is  necessary  to  utter  a  warning  not 
to  expect  too  much,  for  this  method  of  inquiry  suffers 
from  certain  very  serious  difficulties  and  disad- 
vantages. 

(1)  Many  types  of  animals  and  plants  are  not 
capable  of  preservation  in  the  fossil  state,  or  else  are 
so  extremely  rare  and  are  found  separated  by  such 
vast  intervals  of  time,  that  their  remains  are  of  no 
practical  assistance  in  our  inquiry.  It  is  particularly 


EVIDENCE  FROM  PALAEONTOLOGY        83 

unfortunate  that  many  of  these  almost  or  entirely 
missing  types  are  precisely  those  which  would  be 
most  useful  in  the  investigation.  For  any  organism 
to  be  preserved  and  discovered,  it  is  almost  essential 
that  it  should  have  been  fairly  common  and  abund- 
ant when  living  and  that  is  the  probable  reason  why 
the  beginning  of  so  many  genealogical  lines  should  be 
lost  in  obscurity. 

(2)  Of  the  innumerable  fossil  forms  which  have 
been  made  known,  nearly  all  are  but  partially  pre- 
served, only  the  hard  parts,  bones,  teeth,  shells,  etc., 
remaining,  and  the  interpretation  of  these  incom- 
plete remains  is  often  difficult,  sometimes  impossible 
in  the  present  state  of  knowledge.     Exceptions  to 
this  almost  universal  rule  are  so  rare  and  occur  so  late 
in  geological  time,  as  to  be  of  little  real  assistance. 

(3)  In  Darwin's  "Origin  of  Species"  is  a  famous 
chapter  devoted  to  an  examination  of  "the  imperfec- 
tion of  the  geological  record,"  in  which  he  points  out 
the    accidental    and   haphazard  way  in  which  or- 
ganisms have  been  fossilized  and  the  great  gaps  which 
occur  in  the  history  of  life  on  the  earth,  so  far  as  that 
history  has  been  discovered  and  deciphered.    "I  look 
at  the  geological  record  as  a  history  of  the  world  im- 
perfectly kept  and  written  in  a  changing  dialect;  of 
this  history  we  possess  the  last  volume  alone,  relating 
only  to  two  or  three  countries.    Of  this  volume  only 
here  and  there  a  short  chapter  has  been  preserved; 
and  of  each  page  only  here  and  there  a  few  lines. 
Each  word  of  the  slowly  changing  language,  more  or 


84  THE  THEORY  OF  EVOLUTION 

less  different  in  the  successive  chapters,  may  repre- 
sent the  forms  of  life  which  are  entombed  in  our 
successive  formations  and  which  falsely  appear  to  us 
to  have  been  abruptly  introduced."  * 

An  excellent  example  of  the  imperfectly  recorded 
facts  is  afforded  by  the  long  and  narrow,  but  very 
thick  belt  of  rocks,  which  forms  the  surface  of  the 
Connecticut  Valley  and  runs  unbrokenly  from  the 
Hudson  River  across  New  Jersey,  Pennsylvania  and 
Maryland  into  Virginia;  the  whole  formation  is 
referred  to  what  geologists  call  the  Triassic  period. 
These  rocks,  which  were  not  laid  down  in  the  sea, 
but  on  land  and  in  various  bodies  of  fresh  water, 
are  remarkably  barren  of  fossils  and  one  may  search 
through  great  thicknesses  and  over  weary  miles  with- 
out finding  a  trace  of  ancient  life.  In  several  local- 
ities, notably  in  western  Massachusetts,  were  exten- 
sive mud-flats,  which  have  preserved  countless 
foot-prints  of  the  great  variety  of  reptiles  then 
inhabiting  North  America;  in  size,  these  impressions 
range  from  the  prints  made  by  tiny  creatures,  no 
larger  than  a  sparrow,  to  monsters  which  left  tracks 
each  eighteen  inches  long.  From  the  diversity  in 
form  of  the  tracks  it  is  obvious  that  a  very  large 
variety  of  animals  must  have  made  them.  Yet  of  all 
this  host,  bones  of  only  two  or  three  have  been  found, 
in  localities  scattered  from  Pennsylvania  to  Massa- 
chusetts and,  when  it  is  remembered  that  these 
rocks  are  quarried  very  extensively  for  building  stone, 

1  Origin  of  Species,  6th  Ed.,  London,  1872,  p.  289. 


EVIDENCE  FROM  PALAEONTOLOGY        85 

the  paucity  of  fossils  is  all  the  more  evident.  It 
needs  no  argument  to  prove  that  in  this  case  the 
record  is  lamentably  incomplete  and  it  is  typical  of  a 
great  many  others.  In  a  sense,  the  foot-prints  them- 
selves constitute  a  record,  but  it  is  one  that  we  do 
not  know  how  to  decipher. 

On  the  other  hand,  matters  have  greatly  improved 
since  Darwin  wrote  his  oft-cited  Chapter  X;  many 
lands  then  geologically  unknown  have  been  explored 
and  many  of  the  missing  chapters  and  paragraphs  in 
the  history  of  life  have  been  brought  to  light.  The 
most  ancient  biologically  intelligible  period  of  the 
earth's  history  is  called  the  Cambrian  and,  compared 
with  the  succeeding  periods,  the  Cambrian  has  al- 
ways been  poor  in  fossils,  great  areas  and  thicknesses 
of  rocks  being  entirely  barren.  No  one  could  doubt 
that  our  knowledge  of  Cambrian  life  was  most  in- 
complete and  inadequate.  A  few  years  ago  Dr.  C.  D. 
Walcott,  Secretary  of  the  Smithsonian  Institution, 
discovered  in  the  Canadian  Rockies  a  most  marvel- 
lous series  of  Cambrian  fossils  of  an  incredible  del- 
icacy and  beauty  of  preservation,  which  have  thrown 
a  flood  of  new  and  unexpected  light  into  very  dark 
places.  It  is  clear  that  the  Cambrian  seas  swarmed 
with  a  great  variety  and  profusion  of  life,  but  that  in 
only  a  few  places,  so  far  known  to  us,  were  the  condi- 
tions such  that  these  delicate  creatures  could  be 
preserved.  It  is  not  possible  to  say  how  far  the 
difficulty  caused  by  the  imperfection  of  the  geological 
record  will  be  removed  by  the  progress  of  discovery. 


86  THE  THEORY  OF  EVOLUTION 

Even  as  matters  stand  to-day,  the  astonishing  fact  is 
that  so  much  has  been  preserved,  rather  than  that  the 
story  is  so  incomplete.  Notwithstanding  all  the 
difficulties,  the  palseontological  method  remains  one 
of  the  most  valuable  means  of  testing  the  theory  of 
evolution,  because  certain  chapters  in  the  history  of 
life  have  been  recorded  with  a  minuteness  that  is 
really  very  surprising. 

In  the  brief  time  at  my  disposal  it  is  quite  im- 
practicable to  make  clear  all  the  geological  pre- 
liminaries which  are  needed  to  explain  the  facts  of 
palaeontology.  It  may  seem  incredible  that  a  fossil 
found  a  thousand  feet  below  the  surface  is  the  re- 
mains of  a  creature  that  once  swam  in  the  sea  and 
that  it  was  buried  in  what  was  the  sea-bottom  at  the 
time  of  its  death.  Yet  such  is  the  fact  and  the  ex- 
planation is  very  simple.  I  must  also  ask  you  to 
take  for  granted  the  possibility  of  arranging  the 
fossils  which  are  buried  in  the  rocks  in  the  chrono- 
logical order  of  their  succession.  This  is  no  assump- 
tion made  to  bolster  up  any  theory,  for,  in  its  main 
outlines,  the  scheme  of  chronology  which  we  now  use 
had  been  worked  out  long  before  the  publication  of 
Darwin's  revolutionary  book,  when  the  theory  of 
special  creation  had  full  possession  of  the  field.  Any 
of  my  hearers  who  may  be  interested  to  learn  how 
this  chronological  succession  of  the  forms  of  life  has 
been  ascertained,  will  find  the  explanation  in  any  of 
the  standard  text-books  of  geology. 

This  arrangement  in  the  order  of  succession  in  time 


EVIDENCE  FROM  PALAEONTOLOGY        87 


is  essential  to  a  comprehension  of  the  huge  mass  of 
facts  which  have  already  been  gathered.  It  is  further 
possible  to  make  an  approximate  determination  of 
the  relative  chronology  of  the  rocks  and  fossils  in 
different  regions  and  continents,  a  determination 
which  is  of  the  utmost  importance,  as  will  appear 
later.  In  the  following  sketch  of  palseontological 
evidence,  it  is  not  practicable  to  avoid  all  mention  of 
the  technical  terms  used  in  designating  the  divisions 
of  geological  time  and  so  a  table  of  these  terms  is 
appended. 


Cenozoic  era 


Mesozoic  era 


Permian  period 

Carboniferous  period 

Devonian  period 
Palaeozoic  era  1  Silurian  period 

Ordovician  period 
[  Cambrian  period 
Pre-Cambrian  eras. 

N.  B.  The  arrangement  of  the  table  is  such  that  the  most  ancient 
times  are  at  the  bottom  and  the  latest  at  the  top;  the  epochs  of  the 
Mesozoic  and  Palaeozoic  eras  are  omitted  as  unnecessary. 

If,  neglecting  all  details,  we  survey  the  great  and 
ever-changing  procession  of  life,  as  it  is  recorded  in 


Quaternary  period 

Recent  epoch 
Pleistocene  epoch 

Pliocene  epoch 

Miocene  epoch 

Tertiary  period 

Oligocene  epoch 

Eocene  epoch 

Paleocene  epoch 

1  Cretaceous  period 

j  Jurassic  period 

[  Triassic  period 

88  THE  THEORY  OF  EVOLUTION 

the  rocks,  the  immediate  impression  received  is  that 
of  constant,  though  not  always  uniform  advance  and 
progress  from  the  most  ancient  times,  and,  the  more 
nearly  the  present  is  approached  in  time,  the  closer  is 
the  approximation  to  the  modern  animal  and  vege- 
table worlds.  This  most  obvious  and  striking  fact 
impressed  itself  upon  the  earlier  palaeontologists,  like 
Cuvier  and  Agassiz,  who  upheld  the  theory  of  special 
creation  and  the  immutability  of  species,  and  they 
interpreted  it  as  the  carrying  out  of  a  systematic 
creative  plan,  an  interpretation  which  is  not  at  all 
invalidated  by  the  acceptance  of  the  evolutionary 
theory. 

The  latest  of  the  eras,  the  Cenozoic,  in  which  we 
are  now  living,  is  characterized  by  a  land  vegetation 
consisting  chiefly  of  the  flowering  plants,  such  as  the 
trees,  bushes  and  herbs,  with  which  we  are  all  famil- 
iar, also  palms,  lilies  and  grasses.  The  conifers,  or 
"evergreens,"  pines,  spruces,  firs,  etc.,  also  belong  to 
the  flowering  plants,  but  to  a  lower  grade,  but  of  the 
non-flowering  or  cryptogamic  plants,  only  the  ferns, 
mosses  and  lichens  can  be  called  abundant;  the  fungi 
and  microscopic  plants  are  left  out  of  account,  as  so 
little  is  known  of  their  history.  Certain  groups  which 
contain  but  few  species  are  of  interest  as  being  the 
last  survivors  of  what  were  once  great  and  dominant 
assemblages;  the  Cycads,  or  erroneously  so-called 
"Sago  Palms,"  are  now  confined  to  the  warmer  parts 
of  the  earth  and,  among  the  cryptogamic  plants,  the 
little  ground-pines,  or  lycopods,  and  the  horse-tails 


EVIDENCE  FROM  PALAEONTOLOGY         89 

are  remnants  of  a  more  ancient  vegetation.  Among 
animals,  the  most  conspicuous  groups  are  the  mam- 
mals, or  warm-blooded  quadrupeds,  and  the  birds; 
the  reptiles  occupy  a  very  subordinate  position  and 
only  five  orders  of  them  are  found,  lizards  and 
snakes,  turtles  and  crocodiles,  including  a  single 
lizard-like  creature  which  is  confined  to  New  Zealand. 
The  overwhelming  majority  of  Cenozoic  fishes  be- 
long to  the  sub-class  of  the  bony  fishes  or  teleosts, 
and  of  the  remainder  almost  all  are  sharks.  But 
there  are,  almost  exclusively  in  fresh  waters  and 
consisting  of  very  few  forms,  two  groups  of  very 
ancient  date,  the  ganoids,  and  the  lung-fishes  of  the 
southern  hemisphere.  Of  the  vast  host  of  Cenozoic 
invertebrates  it  is  impossible  to  make  the  most  hasty 
survey  and  nothing  more  can  be  attempted  than  to 
mention  a  few  of  the  most  conspicuous  kinds.  There 
are  countless  insects,  especially  of  the  higher  orders, 
beetles  (Coleoptera),  moths  and  butterflies  (Lepi- 
doptera),  ants,  bees,  wasps  (Hymenoptera)  and  flies 
(Diptera).  There  is  a  great  profusion  of  marine 
life,  crustaceans,  molluscs,  star-fishes,  sea-urchins, 
and  others  innumerable;  survivals  of  formerly  abun- 
dant groups  are  the  lamp-shells,  or  brachiopods,  and 
the  sea-lilies  or  crinoids. 

While  this  general  statement  will  serve  to  give 
some  conception  of  Cenozoic  life  as  a  whole,  the 
periods  and  epochs  of  that  era  are  each  char- 
acterized by  stages  of  constant  progress,  which 
may  sometimes,  when  conditions  are  favourable, 


90  THE  THEORY  OF  EVOLUTION 

be  followed  out  in  surprising  detail.  Indeed,  the 
changes  within  the  eras  and  periods  are  as  marked 
as  those  which  distinguish  these  divisions  from  one 
another. 

The  vegetation  of  the  latest  part  of  the  Mesozoic 
era  is  substantially  the  same  as  that  of  the  Cenozoic, 
the  great  revolution  in  plant  life  taking  place  within 
the  Cretaceous  period,  but  the  characteristic  Meso- 
zoic plants  were  conifers  and  cycads,  the  latter  in 
great  variety  and  spread  all  over  the  earth,  even 
to  the  Antarctic  continent.  Birds  are  rare  and  far 
more  primitive  than  those  of  the  Cenozoic,  and  the 
same  is  true  of  the  mammals,  all  of  which  were  tiny 
creatures  of  very  low  grade.  The  conspicuous  and 
all-dominating  group  was  that  of  the  reptiles,  so 
that  the  Mesozoic  is  appropriately  called  the  "Age 
of  Reptiles."  These  creatures  swarmed  in  the  sea,  on 
the  land  and  in  the  air  and  many  attained  gigantic 
size,  incomparably  the  largest  land  animals  that 
ever  lived.  The  contrast  between  the  Cenozoic  and 
Mesozoic  is  well  summarized  by  the  fact  that  while 
in  the  Cenozoic  only  five  orders  of  reptiles  have 
been  found,  in  the  Mesozoic  there  were  no  less 
than  twenty.  Like  the  modern  type  of  vegetation, 
the  bony  fishes  came  in  in  the  Cretaceous,  while 
before  that  period  the  fish-fauna  was  almost  en- 
tirely composed  of  ganoids  and  sharks.  Many  of 
the  Mesozoic  invertebrates  were  of  modern  type, 
but  there  were  several  highly  characteristic  groups. 
Among  the  Mollusca  the  bivalves  and  gastro- 


EVIDENCE  FROM  PALAEONTOLOGY        91 

pods  were  less  abundant  and  varied  than  in  the 
Cenozoic,  the  beautiful  chambered  shells  called  Am- 
monites were  incredibly  numerous  and  diversified. 
In  the  successive  forms  of  these  shells  many 
striking  genealogical  series  have  been  made  out. 
The  curious  Belemnites  are  almost  exclusively 
Mesozoic. 

In  all  the  recorded  history  of  life  there  was  no  such 
radical  and  far-reaching  revolution  as  that  between 
the  Palaeozoic  and  Mesozoic  eras.  Could  a  naturalist 
be  transported  into  the  former  he  would  find  him- 
self in  an  unfamiliar  world,  in  which  almost  every 
animal  and  plant  that  he  saw  would  be  strange  to 
him.  The  land  vegetation  was  made  up  of  gigantic, 
tree-like  cryptogams,  especially  the  lycopods  and 
horse-tails,  and  the  abundant  ferns  were  both  in 
arboreal  and  herbaceous  forms.  The  flowering  plants 
were  represented  but  scantily  and  by  curious  trees 
allied  to  the  cycads  and  conifers,  but  of  such  plants 
in  the  ordinary  use  of  the  word,  there  was  no  trace, 
nor  was  any  mammal  or  bird  in  existence.  The  vast 
majority  of  Palaeozoic  fossils  belong  to  marine  in- 
vertebrates, for,  except  in  the  latter  part  of  the  era, 
the  only  vertebrates  were  fishes  and  fish-like  forms, 
sharks,  ganoids,  lung-fishes  and  certain  bizarre, 
mailed  animals,  which  were  below  the  fishes  in  the 
scale  of  organization.  The  seas  were  swarming  with 
a  profusion  of  invertebrate  life,  many  of  which, 
though  referable  to  types  and  classes  which  are  still 
existing,  were  much  more  primitive  in  structure 


92  THE  THEORY  OF  EVOLUTION 

than  any  living  representatives  of  those  groups, 
and  groups  which  now  are  very  rare  were  then  ex- 
tremely abundant,  and  many  were  dominant  in  the 
Palaeozoic.  For  example,  molluscs  were  less  com- 
mon and  brachiopods  and  sea-lilies  vastly  more  so 
then  than  now.  Confined  to  the  Palaeozoic  were 
the  remarkable  Trilobites,  an  extinct  sub-class  of 
the  Crustacea,  while  the  higher  groups  of  the 
latter  were  wanting.  Spiders  and  scorpions  were 
followed  by  insects  belonging  to  the  lower  and  more 
primitive  divisions,  such  as  cockroaches  and  dragon- 
flies. 

The  species  of  Palaeozoic  fossils  already  described 
are  numbered  by  thousands  and  the  number  is 
constantly  growing,  as  new  discoveries  are  made. 
Any  attempt  to  survey  this  vast  assemblage  is  a 
highly  technical  problem  and  cannot  be  made  here. 
The  few  salient  features  which  have  been  selected 
are  enough  to  show  that  the  farther  back  in  time 
we  explore  the  history  of  life,  the  greater  do  the 
differences  from  the  present  order  of  things  become 
and  that  not  in  any  haphazard  way,  but  by  changes 
in  a  well-defined  serial  order.  The  history  of  life, 
both  animal  and  vegetable,  is  a  story  of  progress  and 
differentiation,  of  advance  continued  through  mil- 
lions of  years  to  modern  conditions  from  far-off 
beginnings,  which  were  of  radically  different  charac- 
ter. In  itself,  this  story  is  a  striking  testimony  to 
the  theory  of  evolution  and  the  testimony  is  much 
strengthened,  when  the  material  at  hand  permits  a 


EVIDENCE  FROM  PALEONTOLOGY        93 

close   study  of   the  genealogy  of   some  restricted 
group. 

It  has  often  been  objected  that  the  palseontological 
record,  as  we  actually  have  it,  is  irreconcilable  with 
the  evolutionary  conception,  because  of  the  many 
cases  of  the  sudden  and  unheralded  appearance  of 
new  kinds  of  organisms,  such  as  the  remarkable 
outburst  of  modern  types  of  plants  and  fishes  in 
the  upper  Cretaceous  period.  This  objection  over- 
looks the  phenomena  of  migration,  the  importance 
of  which  has  only  lately  come  to  be  recognized;  the 
many  geographical  changes  in  the  connections  be- 
tween the  different  continents  and  between  the 
seas,  which  have  demonstrably  occurred  in  the 
past,  have  from  time  to  time  altered  the  possibilities 
and  directions  of  migration.  Hence,  the  appear- 
ance in  any  given  area  of  new  forms  may  either  be 
through  the  modification  of  older  forms  which  pre- 
viously inhabited  that  area,  or  by  immigration  from 
some  other  region.  It  is  therefore  not  to  be  taken 
for  granted  that  any  group  originated  in  the  region 
where  we  happen  first  to  find  it;  quite  as  frequently 
it  arose  in  some  other  region  and  spread  gradually 
from  that.  A  very  instructive  illustration  of  this 
principle  is  afforded  by  the  history  of  the  elephants, 
which  appeared  suddenly  (lower  Miocene)  in  Europe 
and  somewhat  later  (middle  Miocene)  in  North 
America.  In  neither  continent  has  anything  been 
found  in  any  preceding  formation,  which  could  plausi- 
bly be  regarded  as  ancestral  to  these  ancient  and 


94  THE  THEORY  OF  EVOLUTION 

primitive  elephants,  which,  to  all  appearances,  might 
have  risen  up  out  of  the  ground,  as  Buffon  fancied 
that  they  actually  did.  However,  on  evolutionary 
principles,  they  must  have  had  ancestors  and  the 
obvious  inference  was  that  they  had  immigrated 
into  the  northern  continents  from  some  southern 
land.  By  a  process  of  elimination,  the  conclusion 
was  reached  that  Africa  must  have  been  the  original 
home  of  these  animals,  a  result  which  has  been 
amply  confirmed  by  discoveries  made  not  long  ago 
in  Egypt,  which  carry  the  history  of  the  order  much 
farther  back  in  time  than  had  previously  been  pos- 
sible. 

Another  objection  which  has  frequently  been 
brought  against  the  palseontological  argument  for 
the  theory  of  evolution  is  that,  while  the  genealogical 
series  made  out  within  certain  families  may  be  ad- 
mitted as  proving  development  within  relatively 
small  groups,  the  fossil  record  fails  to  connect  the 
larger  and  more  widely  separated  types,  thus  indi- 
cating that  evolution,  while  real  enough,  is  of  strictly 
limited  possibilities.  This,  it  will  be  remembered, 
is  the  same  objection  as  has  been  urged  against  the 
argument  from  embryology,  and  the  answer  is  in 
both  cases  the  same.  In  part,  the  objection  is 
founded  upon  an  assumption  which  is  not  true  and 
is  being  gradually  refuted  by  the  progress  of  dis- 
covery and,  where  this  is  not  likely  to  happen,  there 
are  obvious  reasons  why  we  cannot  expect  to  find 
the  connecting  links.  The  most  ancient  known 


EVIDENCE  FROM  PALAEONTOLOGY        95 

fossiliferous  rocks  show  that  most  of  the  principal 
types  of  animal  life  were  already  differentiated  at 
the  time  when  those  rocks  were  formed  and  hence 
there  is  little  hope  of  finding  the  beginnings  of  those 
types.  It  must  not  be  forgotten  that  there  is  abun- 
dant indirect  evidence  that  life  had  existed  on  the 
earth  through  unimaginably  long  ages  before  the 
formation  of  the  earliest  fossiliferous  rocks;  that  is 
what  Darwin  meant  when  he  said  that  the  geological 
record  contained  only  the  last  volume  of  the  world's 
history. 

An  admirable  illustration  of  the  manner  in  which 
the  evidence  of  fossils  may  serve  to  show  the  con- 
nection between  types  which  are  very  widely  sepa- 
rated in  the  modern  world,  is  afforded  by  the  vegeta- 
tion of  the  Carboniferous  period,  the  last  but  one 
of  the  Palaeozoic  divisions.  This  luxuriant  vegeta- 
tion was  composed  almost  entirely  of  non-flowering 
or  cryptogamic  plants  and,  in  addition  to  a  great 
variety  of  ferns,  giant  lycopods  and  horse-tails,  there 
were  two  groups  of  especial  interest  to  evolutionists: 
(1)  the  Sphenophyllales,  very  slender,  probably 
trailing  and  climbing  plants,  which  are  intermediate 
in  structure  between  the  two  cryptogamic  classes, 
the  lycopods  and  horse-tails.  Doubtless,  they  were 
the  survivors  in  Carboniferous  times  of  more  ancient 
plants  which  were,  in  turn,  the  common  ancestors 
of  the  two  classes  named.  (2)  A  proverbially  im- 
passable gulf  yawns  between  the  flowering  and  the 
cryptogamic  plants  and  yet  the  gulf  is  at  least  par- 


96  THE  THEORY  OF  EVOLUTION 

tially  bridged  by  the  Carboniferous  group  of  the 
Cycadofilices,  which,  as  the  name  indicates,  serve 
to  connect  the  cycads  with  the  ferns;  in  ap- 
pearance of  stem  and  foliage  the  Cycadofilices  most 
resembled  ferns.  In  view  of  such  facts,  it  is  idle 
to  say,  as  is  still  so  often  said,  that  the  fossils 
have  never  afforded  transitions  between  groups 
which  are  now  radically  distinct  and  widely  sepa- 
rated. 

Another  very  interesting  and  significant  transition 
is  that  displayed  by  certain  fossil  birds,  which  clearly 
indicate  their  derivation  from  reptiles.  The  relation- 
ship between  birds  and  reptiles  is  shown  by  com- 
parative anatomy  and  embryology  and  is  strongly 
confirmed  by  the  blood  tests  which  were  described 
in  the  preceding  lecture.  To  this  mass  of  testimony 
from  three  independent  lines  of  inquiry,  palaeontology 
adds  its  quota.  At  the  famous  quarries  of  litho- 
graphic stone  at  Solenhofen,  in  Bavaria,  has  been 
found  a  marvellously  preserved  record  of  the  later 
Jurassic  life  of  Europe  and  here  were  discovered  two 
nearly  complete  skeletons  of  the  most  ancient  known 
bird,  belonging  to  the  genus  Archceopteryx.  Though 
an  unmistakable  bird,  Archceopteryx  yet  retained 
many  characteristics  of  its  reptilian  ancestry,  char- 
acters which  are  repeated  in  no  existing  bird,  or 
appear  only  as  transitory  features  in  the  course  of 
embryological  development.  There  is  no  horny 
beak,  but  the  jaws  are  provided  with  numerous 
small  teeth,  which  explains  the  significance  of  the 


EVIDENCE  FROM  PALAEONTOLOGY        97 

formation  of  tooth-germs  in  certain  embryo  birds, 
as  was  pointed  out  in  Lecture  III  (see  p.  71).  The 
joints  of  the  backbone  lack  the  characteristic  fea- 
tures of  modern  birds,  and  are  much  more  reptilian 
in  structure.  A  very  remarkable  feature  is  the  long, 
lizard-like  tail,  com- 
posed of  many  vertebrae, 
to  each  of  which  a  pair 
of  quill  feathers  is  at- 
tached. The  hand  is 
most  un-birdlike;  its 
four  fingers  are  all  sepa- 
rate and  free  from  one 
another  and  are  com- 
pletely developed,  with 
the  same  number  of 
joints  as  the  correspond- 
ing fingers  of  a  lizard, 
and  each  one  is  pro- 
vided With  a  Claw.  FlG  6  Restoration  of  Archaopteryx, 
There  are  feathers  On  Jurassic  of  Bavaria.  (After  An- 
.,  .  .,  j  dreae). 

the     wings,     tail     and 

legs,  but    the   head,    neck    and  body   are   mostly 

naked.   We  may  well  hope  that  the  ancestors  of 

Archceopteryx,    fully    intermediate    between    birds 

and   reptiles,   will   yet   be   discovered   in   Triassic 

rocks. 

The  ancestry  of  mammals  is  still  a  question  of 
much  uncertainty  and  zoologists  continue  to  debate 
whether  they  were  derived  from  reptiles  or  amphib- 


98  THE  THEORY  OF  EVOLUTION 

ians,  a  state  of  affairs  which  Professor  Steinmann 
regards  as  so  scandalous  to  science  (see  p.  4),  but 
there  are  good  reasons  for  the  uncertainty.  Mesozoic 
mammals  are  very  rare  and,  though  they  have  been 
found  in  three  continents,  the  specimens  are  all  so 
incomplete  that  they  contribute  but  little  to  a  solu- 
tion of  the  problem.  Certain  reptiles  obtained  from 
the  Permian  rocks  of  South  Africa  decidedly  sug- 
gest that  they  were  closely  allied  to  the  long  sought 
ancestors  of  the  mammals,  but  the  gap  is  still  too 
great  for  any  final  decision. 

The  number  of  genealogical  or  phylogenetic  series 
within  narrower  limits,  which  has  already  been  made 
out,  would  fill  many  volumes.  These  genealogies 
have  been  determined  in  a  great  many  different 
types  of  animals,  both  vertebrate  and  invertebrate, 
but  I  shall  confine  attention  to  the  vertebrates,  as 
it  is  easier  to  make  these  intelligible,  and  can  select 
only  a  few  of  the  most  conspicuous  and  best  known 
instances.  So  long  as  it  is  clearly  understood  that 
but  a  small  number  of  illustrative  examples  are  here 
set  forth,  it  will  serve  no  good  purpose  to  pile  up  a 
great  number  of  more  or  less  similar  cases.  Some 
of  the  best  preserved  and  most  complete  of  the 
phylogenetic  series  are  among  the  families  of  mam- 
mals, the  changes  in  which  may  be  traced  through 
the  successive  stages  of  the  Tertiary  period.  The 
most  complete  of  all  the  series  so  far  discovered  is 
that  which  displays  the  development  of  the  horses 
in  North  America.  One  cannot  but  hesitate  to  tell 


EVIDENCE  FROM  PALAEONTOLOGY        99 

again  this  oft-told  tale,  which  has  been  reiterated 
until  it  has  become  a  hackneyed  commonplace. 
Friends  and  foes  of  evolution  are  alike  given  to 
sneering  at  it  and  calling  it  the  "parade  horse,"  the 
"hobby  horse,"  the  "stalking  horse,"  and  other 
opprobrious  names,  seeming  almost  to  imagine  that, 
if  testimony  be  very  often  repeated,  it  loses  all 
validity.  Logically,  this  is  on  a  par  with  the  pro- 
cedure of  the  ship's  crew  in  the  "Hunting  of  the 
Snark,"  who  believed  that,  by  repeating  a  state- 
ment three  times,  they  proved  it.  On  the  other 
hand,  no  sketch,  however  slight,  which  purports  to 
give  an  outline  of  the  evidences  of  evolution,  can  in 
fairness  omit  all  mention  of  this  remarkable  case. 

The  Recent  and  Pleistocene  horses,  including  in 
that  term  not  only  the  true  horses,  but  also  the 
asses,  zebras,  etc.,  have  a  skeleton  which  is  wonder- 
fully adapted  to  swift  running,  while  their  teeth  are 
highly  specialized  and  elaborated  for  the  masticating 
of  grasses,  which  are  among  the  most  abrasive  of 
plants.  The  front  teeth  (incisors)  used  for  cropping, 
continue  to  grow  for  a  long  period  and  have  on  their 
biting  surfaces  a  deep  pit,  lined  with  enamel,  which 
horsemen  call  the  "mark."  The  grinding  teeth  are 
very  high-crowned,  growing  persistently  in  height, 
as  they  are  worn  down,  to  an  advanced  period  of 
life,  when  the  formation  of  roots  puts  an  end  to 
growth;  they  have  masticating  surfaces  which  are 
kept  rough  by  the  different  hardness  of  the  three 
substances  exposed  on  these  surfaces,  dentine, 


100  THE  THEORY  OF  EVOLUTION 

enamel  and  cement,  which  wear  unequally.  The 
arrangement  of  the  three  tooth-substances  is  in  a 
highly  characteristic  and  very  complex  pattern,  and 
the  anterior  grinding  teeth  (premolars)  are  similar 
in  form  and  size  and  in  the  pattern  of  the  masticat- 
ing surface  to  the  posterior  teeth  (molars).  The 
facial  portion  of  the  skull  is  very  long  and  the  jaws 
high  vertically,  to  provide  room  for  the  very  deep 
tooth-sockets.  The  eye-socket  is  completely  en- 
circled in  bone  and  displaced  backward  behind  the 
teeth,  which  otherwise  would  invade  the  socket  and 
compress  the  eyeball. 

The  neck  is  long  and  its  vertebrae  are  much  special- 
ized to  secure  the  greatest  freedom  of  movement  in 
combination  with  strength.  The  limbs  and  es- 
pecially the  feet  are  elongate  and  their  bones  have 
undergone  nearly  the  utmost  possible  reduction  in 
number,  either  by  complete  suppression  and  loss,  or 
by  co-ossification,  such  concentration  being  favour- 
able to  rapidity  of  movement;  we  find  it  repeated  in 
other  swift  runners,  such  as  antelopes  and  deer. 
The  external  bone  of  the  forearm  (ulna)  is  greatly 
reduced  in  thickness,  the  middle  portion  of  its  shaft 
has  been  lost  and  its  two  ends  are  co-ossified  with 
the  enlarged  radius,  which  carries  the  whole  weight 
imposed  upon  the  fore  leg.  Similarly,  in  the  hind 
leg  only  the  shin-bone  (tibia)  is  left  in  the  lower 
segment,  while  the  fibula  seems  to  have  completely 
disappeared,  but  if  the  skeleton  of  a  young  colt  be 
examined,  it  is  seen  that,  while  the  whole  shaft  of 


EVIDENCE  FROM  PALAEONTOLOGY       101 

the  fibula  has  been  suppressed,  its  two  extreme  ends 
remain  and  in  the  adult  are  indistinguishably  fused 
with  the  tibia.  The  feet  are  very  long  and  have  but 
a  single  functional  toe  each,  which  is  greatly  en- 
larged and  strengthened  to  bear  the  heavy  weight. 
This  remaining  toe  is  the  median  one,  or  third,  of 
the  original  five,  and  the  splint-bones  are  remnants 
of  the  second  and  fourth,  but  have  no  joints  or 
hoofs  attached  to  them,  and  are  not  visible  in  the 
living  animal.  The  horses  walk  on  the  very  tips 
of  their  toes  and  the  curious,  box-like  hoof,  which 
encloses  the  terminal  joint  of  the  single  functional 
toe,  is  highly  characteristic. 

In  surveying  the  history  of  the  changes  which 
lead  back  from  the  Recent  horses  to  their  far  distant 
ancestors  of  the  lower  Eocene,  it  will  be  necessary 
to  omit  most  of  the  stages,  for  lack  of  time,  and 
mention  only  the  most  obvious.  Passing  back  to 
the  lower  Pliocene  and  upper  Miocene,  we  find  that 
the  horses  of  those  times,  while  unmistakably  equine, 
differed  in  certain  very  significant  ways  from  their 
modern  descendants.  They  are  decidedly  smaller 
and  of  more  slender  and  deer-like  proportions;  their 
grinding  teeth  are  hardly  more  than  half  as  high 
proportionately  and  with  masticating  surfaces  of 
similar,  but  less  complicated  pattern.  The  feet  are 
three-toed,  the  lateral  toes  (second  and  fourth) 
having  a  full  complement  of  joints  and  carrying 
hoofs,  but  were  more  dew-claws,  not  reaching  the 
ground,  and  could  have  been  of  service  only  in  soft 


102 


THE  THEORY  OF  EVOLUTION 


mud.    A  small,  nodular  rudiment  of  the  fifth  toe  is 
also  present.     The  horses  of  the  middle  and  lower 


FIG.  7.  Four  stages  in  the  evolution  of  the  equine  skull.  A,  Eohippus, 
lower  Eocene.  B,  Mesohippus,  lower  Oligocene.  C,  Protohippua, 
upper  Miocene.  D,  Equus,  Pleistocene  and  Recent. 

Miocene  display  the  transition  between  the  high- 
crowned,  cement-covered  grinding  teeth  and  those 


EVIDENCE  FROM  PALEONTOLOGY       103 

with  low  crowns,  which  were  incapable  of  growth 
after  they  had  cut  the  gum,  and  have  no  cement  on 
the  crown,  but  only  on  the  roots. 

It  is  very  interesting  to  note  that  this  adoption  of 
a  high-crowned,  persistently  growing  type  of  grind- 
ing teeth  was  by  no  means  confined  to  the  horses, 
but  was  repeated  in  several  other  families  of  hoofed 
animals  and  gnawers  (Rodentia),  and  came  about 
during  the  Miocene  epoch.  This  transformation  of 
the  teeth  indicates  a  change  from  the  habit  of 
browsing  upon  leaves  and  other  soft  vegetable  sub- 
stances to  grazing,  or  feeding  upon  the  hard,  abrasive 
grasses,  and  it  is  highly  suggestive  that  this  coincided 
with,  or  quickly  followed  the  spread  of  grassy  plains, 
which  afforded  a  new  and  abundant  supply  of  nu- 
tritious food. 

Again  omitting  intermediate  stages  and  taking 
another  long  backward  step,  we  come  to  the  horses 
of  the  lower  Oligocene,  in  which  are  notable  changes 
in  all  parts  of  the  skeleton,  as  well  as  a  distinct  reduc- 
tion in  size,  for  these  animals  are  no  larger  than  sheep. 
The  teeth  are  low-crowned  and  devoid  of  cement, 
save  on  the  roots,  and  the  sharp-edged  incisors  have 
no  mark.  The  facial  portion  of  the  skull  is  shorter 
and  the  eye-socket  farther  forward,  placed  over 
the  teeth,  and  is  partially  open  behind;  the  jaws  are 
shallow  and  slender,  as  there  is  no  need  of  providing 
for  very  deep  sockets.  The  neck  is  only  moderately 
elongate  and  its  vertebrae  are  much  less  specialized. 
In  the  forearm  the  ulna  is  already  much  reduced  in 


104  THE  THEORY  OF  EVOLUTION 

thickness,  but  is  complete  and  separate  from  the 
radius,  and  in  the  lower  hind  leg,  the  fibula  is  separate 
and  has  a  very  slender  shaft,  but  it  is  entire  and 
uninterrupted.  The  feet  are  still  three-toed,  but 
the  lateral  toes  are  longer  and  more  useful  in  carry- 
ing weight. 

Once  more  omitting  many  intermediate  forms, 
we  may  examine  the  most  ancient  known  horses, 
those  of  the  lower  Eocene,  which  were  little  creatures, 
not  exceeding  a  fox  in  size.  The  grinding  teeth  have 
a  very  simple  and  primitive  pattern,  though  the 
beginnings  of  the  complicated  horse-pattern  may  be 
discerned  in  them;  the  premolars  are  all  smaller  and 
simpler  than  the  molars.  The  neck  and  limbs  and 
especially  the  feet  are  relatively  short;  there  are  four 
functional  toes  in  the  front  foot  and  three  in  the 
hind,  but  the  latter  also  has  splint-like  rudiments 
of  the  first  and  fifth,  plainly  indicating  derivation 
from  a  five- toed  foot. 

Here  the  line  breaks  off  and  cannot  at  present  be 
traced  farther  back,  the  preceding  Paleocene  of 
North  America  having  yielded  nothing  that  can  be 
regarded  as  ancestral  to  this  family.  The  lower 
Eocene  horses  in  Europe  are  of  the  same  type  and 
there  also  they  appeared  unheralded  by  any  pre- 
decessors in  the  Paleocene.  The  obvious  conclusion 
from  these  facts  is  that  the  family  arose  in  some 
region,  as  yet  unidentified,  but  which  not  improbably 
was  central  Asia,  and  thence  migrated  into  Europe 
and  North  America;  the  latter  continent  was  made 


FIG  8  Four  stages  in  the  evolution  of  the  equine  fore  foot.  A,  Eohippua, 
lower  Eocene.  B,  Mesohippus,  lower  Oligocene.  C,  Protohippus,  upper 
Miocene,.  D,  Equus,  Pleistocene  and  Recent. 


106  THE  THEORY  OF  EVOLUTION 

accessible  by  a  junction  with  northeastern  Asia, 
where  now  are  Bering  Strait  and  Sea. 

Though  the  equine  family  did  not  originate  in 
North  America,  its  principal  development  took  place 
here  and  went  on  unbrokenly  through  the  long  ages  of 
the  Tertiary  period.  After  they  had  arrived  at  a 
high  stage  of  advance  and  differentiation,  the  horses 
spread  to  other  continents  and  eventually  reached  all 
of  them  except  Australia.  Invading  South  America 
in  the  late  Miocene  or  early  Pliocene,  after  the  two 
continents  had  been  joined  by  the  upheaval  of  the 
Isthmus  of  Panama,  they  there  gave  rise  to  a  number 
of  peculiar  and  characteristic  forms.  In  the  Pleis- 
tocene epoch  great  herds  of  horses,  belonging  to  no 
less  than  ten  distinct  species,  ranged  through  the 
forests  and  over  the  plains  of  North  America,  extend- 
ing from  the  Atlantic  to  the  Pacific  and  from  Alaska 
through  Mexico.  In  size,  they  varied  from  a  Shet- 
land pony  to  species  exceeding  the  largest  modern 
draught-horses  in  stature.  Then,  for  some  unknown 
reason,  they  died  out  completely  all  over  the  western 
hemisphere,  a  fate  which  was  by  no  means  confined 
to  the  horses  or  to  the  Americas;  the  close  of  the 
Pleistocene  witnessed  over  three-fifths  of  the  land- 
surface  of  the  globe,  the  extinction  of  a  multitude  of 
the  largest  and  most  conspicuous  mammals,  but  the 
causes  of  such  a  tremendous  mortality  can  only  be 
conjectured. 

In  addition  to  the  main  line  of  equine  descent, 
leading  up  to  the  modern  horses,  there  were  several 


EVIDENCE  FROM  PALAEONTOLOGY       107 

side-branches  of  the  equine  stock,  which,  after  a 
longer  or  shorter  duration,  died  out  without  leaving 
descendants.  One  such  branch  was  that  of  the 
browsing  horses,  which  were  abundant  through  the 
Miocene  and  early  Pliocene  of  North  America;  they 
retained  the  low-crowned  teeth  and  the  three-toed 
foot  until  the  end.  The  peculiar  South  American 
forms  were  likewise  side-branches,  given  off  from  the 
main  stem  at  a  later  date;  one  of  these  was  a  small 
mountain  horse,  with  very  short  feet,  especially 
adapted  to  climbing,  like  several  of  the  existing 
mountain  antelopes.  A  very  early  branch,  which 
probably  originated  in  Asia,  diverged  so  widely  from 
the  equine  series  that  it  is  included  in  a  different 
family,  is  that  of  the  Palseotheres,  which  was  abun- 
dant in  the  Eocene  of  Europe,  but  did  not  extend  its 
range  to  the  western  hemisphere. 

We  have  thus  traced  the  horses  back  to  little 
animals,  whose  structure  is  but  remotely  equine  and 
which  are  in  all  respects  so  unlike  the  modern  species, 
that  hardly  any  palaeontologist  would  be  bold  enough 
to  connect  them,  were  it  not  for  the  many  inter- 
mediate stages,  which  join  these  extremes  in  almost 
unbroken  continuity.  No  more  illuminating  example 
of  evolutionary  development  can  be  found  among  the 
geological  records  of  mammalian  life  than  the  history 
of  the  horses,  just  because  well-preserved  specimens, 
exemplifying  all  the  important  steps  of  change,  have 
been  discovered.  In  turn,  this  was  probably  due  to 
the  fact  that,  from  the  beginning  to  the  end  of  their 


108 


THE  THEORY  OF  EVOLUTION 


separate  history,  the  horses  have  been  individually 
abundant  and  thus  have  had  a  much  better  chance  of 


Me. 


m. 

FIG.  9.  Fore  foot  of  Plio- 
hippus,  Pliocene,  hind- 
er view,  showing  the 
very  long  splint-bones 
(Me.  II  and  IV)  and 
rudiment  of  fifth  digit 
(Mc.V)(froinTroxell). 


-HC.JT. 


FIG.  10.  Fore  foot  of  Horse  (Equus) 
posterior  view,  showing  short 
splint-bones.  Wrist-  and  toe-bones 
omitted. 


being  preserved  as  fossils  than  those  of  their  con- 
temporaries which  were  less  common.  But  there  are 
several  other  families  whose  history  is  only  less  com- 


EVIDENCE  FROM  PALEONTOLOGY       109 

pletely  known  than  that  of  the  horses  and  which 
illustrate  similar  principles.  In  several  instances  we 
are  embarrassed  with  riches,  finding  it  difficult,  or 
even  impracticable,  to  select  from  the  many  possi- 
bilities the  forms  which  are  actually  and  properly 
referable  to  a  given  series.  Indeed,  there  is  every 
reason  to  believe  that,  did  we  possess  well-preserved 
specimens  of  every  mammal  that  ever  lived  on  the 
earth,  we  should  be  entirely  unable  to  find  a  clue  that 
would  guide  us  through  the  complex  maze. 

A  family  that  has  been  traced  from  the  Pleistocene 
to  the  Eocene  of  North  America  is  that  of  the  tapirs, 
but,  for  the  most  part,  by  the  aid  of  rare,  fragmentary 
and  therefore  unsatisfactory  materials.  This,  how- 
ever, is  of  less  consequence  in  the  case  of  the  tapirs, 
for  they  are  a  very  conservative  family  and  have 
undergone  relatively  little  change.  There  has  been  a 
moderate  increase  in  size  and  bulk  in  the  successive 
stages,  a  few  changes  in  the  teeth  and  skull  may  be 
noted,  especially  the  modifications  of  the  latter 
caused  by  the  development  of  a  proboscis,  but  other- 
wise the  bony  structure  of  an  Eocene  tapir  differs  but 
little  from  that  of  its  modern  descendants.  In  the 
Pleistocene,  tapirs  extended  from  Pennsylvania  to 
Argentina  and  to  eastern  and  southern  Asia,  but 
they  had  vanished  from  Europe,  where  they  had  lived 
in  the  Miocene  and  Pliocene.  Thus,  at  one  time  or 
another,  they  ranged  all  over  the  northern  hemi- 
sphere; the  significance  of  this  fact  will  appear  in  the 
next  lecture. 


110  THE  THEORY  OF  EVOLUTION 

Another  family,  the  history  of  which  is  far  more 
fully  recorded,  but  much  more  difficult  to  decipher 
and  understand,  is  that  of  the  rhinoceroses.  The 
difficulty  arises  from  the  number  of  divergent 
branches  and  parallel  series  which  developed  within 
the  family  and  the  local  forms  which  arose  in  the 
various  regions  to  which  they  migrated,  for,  with  the 
exception  of  South  America  and  Australia,  every 
continent  has  had  its  rhinoceroses.  Strange  to  say, 
the  available  evidence  points  to  North  America  as 
the  region  of  their  origin,  though  the  proof  of  this  is 
not  entirely  conclusive.  At  all  events,  the  family  has 
been  traced  farther  back  in  time  here  than  in  any 
other  region.  At  a  very  early  stage,  the  family 
divided  into  three  strongly  divergent  branches,  two 
of  which  were  short-lived,  lasting  only  from  the 
middle  Eocene  through  the  lower  Oligocene,  while 
the  third,  that  of  the  true  rhinoceroses,  has  persisted 
to  the  present  day,  but  is  confined  to  the  warmer 
parts  of  Africa  and  southern  Asia.  The  brief  series 
of  the  cursorial  rhinoceroses  was  exclusively  North 
American  and  differed  from  the  other  branches  of  the 
family  in  being  quite  defenceless,  having  neither 
horns  nor  tusks,  and  entirely  dependent  upon  speed 
for  their  safety.  They  were  lightly  built  and  slender, 
long-necked,  long-legged  and  narrow-footed,  with 
three-toed  feet  before  and  behind.  There  is  much 
about  these  curious  animals  that  suggests  relation- 
ship to  the  horses,  but  the  likeness  is  superficial 
and  due  to  cursorial  habits,  for  every  tooth  and  every 


EVIDENCE  FROM  PALEONTOLOGY       111 

bone  in  the  skeleton  declares  its  affinities  with  the 
rhinoceroses. 

The  second  branch,  though  of  American  origin, 
spread  to  the  Old  World  and  has  been  found  in 
France.  The  short,  low  and  very  broad  skull  had  no 
horns,  but  large  tusks  provided  effective  weapons  of 
defence.  In  marked  contrast  to  the  cursorial  type, 
these  animals  were  presumably  aquatic  in  habit  and 
though  rather  low,  were  very  massive  in  build  and 
had  four  toes  in  the  front-foot;  aside  from  the  head, 
they  must  have  been  quite  hippopotamus-like  in 
appearance. 

The  third  branch,  the  true  rhinoceroses,  has  had  a 
far  more  complicated  history,  because  of  the  number 
of  distinct  series  within  the  group,  which  passed 
through  parallel  courses  of  development;  Professor 
Osborn  recognizes  no  less  than  seven  of  these  series  in 
the  Old  and  New  Worlds.  The  terminal  members  of 
three  series  are  the  modern  African,  Indian  and 
Sumatran  species.  While  the  structure  of  the  true 
rhinoceroses  underwent  very  considerable  changes  in 
that  part  of  their  history  of  which  the  record  has  been 
discovered,  the  changes  are  much  less  radical  than 
those  which  took  place  in  the  horses  in  the  same 
length  of  time  and,  for  the  most  part,  they  affected 
the  skull  and  dentition.  The  true  rhinoceroses  are 
characterized  by  the  peculiar  development  of  the 
front  teeth  (incisors  and  canines);  in  the  modern 
African  species  these  teeth  have  been  entirely  sup- 
pressed, but  in  the  Indian  and  Sumatran  and  the 


112  THE  THEORY  OF  EVOLUTION 

great  majority  of  the  extinct  species,  the  peculiarity 
above  mentioned  may  be  observed.  The  upper  jaw 
has  but  one  incisor  on  each  side  and  no  canine;  the 
remaining  incisor  is  chisel-shaped,  with  a  sharp 
cutting  edge;  in  the  lower  jaw  are  two  incisors  on 
each  side,  but  no  canine.  The  first  lower  incisor  is 
very  small  and  can  be  of  little  or  no  real  use,  but  the 
second  is  a  fairly  large  tusk,  which  points  directly 
forward  and  the  Indian  species,  at  least,  uses  them 
effectively  as  weapons.  All  of  the  existing  species, 
as  well  as  most  of  the  fossil  forms,  are  provided  with 
solid  horns,  which  have  no  bony  core;  with  an  excep- 
tion, to  be  noted  below,  the  horns  are  not  paired,  but, 
if  more  than  one  is  present,  they  are  placed  one  be- 
hind the  other  in  the  middle  line  of  the  skull.  The 
skull  is  much  modified  to  form  a  strong  support  for 
the  horns  and  its  upper  profile  is  convex  in  front, 
concave  behind.  The  neck  is  short,  the  body  very 
large,  the  legs  and  feet  short  and  very  heavy,  with 
three  toes  in  each  foot.  It  is  quite  impossible  to  take 
up  here  the  many  series  of  the  true  rhinoceroses;  all 
that  can  be  done  is  to  indicate  very  briefly  the 
changes  that  have  been  noted  in  the  group  as  a  whole. 
Going  back  at  once  to  the  most  ancient  known  forms 
which  can  be  positively  identified  as  true  rhinoceroses, 
we  find  in  the  lowest  Oligocene  of  North  America  very 
interesting  species  which  help  to  connect  the  rhinoc- 
eroses with  other  families  of  their  order  (Perisso- 
dactyla).  In  these  far-off  animals  the  first  upper 
incisor  had  already  assumed  its  trenchant  shape, 


EVIDENCE  FROM  PALAEONTOLOGY       113 

but  is  smaller  than  in  later  forms,  and  is  followed  by 
two  other  incisors  and  a  canine.  The  lower  jaw  also 
has  three  incisors  on  each  side,  but  has  lost  the 
canine.  The  skull  is  without  horns  and  the  skeleton 
is  much  lighter  and  more  tapir-like  in  its  proportions 
and  the  whole  animal  decidedly  smaller  than  the 
existing  rhinoceroses;  the  front  foot  has  four  toes,  but 
the  external  one  (the  fifth  of  the  original  five),  is 
already  much  reduced  in  size  and  evidently  dwindling 
away.  From  the  Oligocene  to  the  Pliocene,  inclusive, 
North  America  had  an  abundance  of  rhinoceroses, 
both  horned  and  hornless,  but  finally  they  disap- 
peared from  the  continent  entirely. 

In  the  uppermost  part  of  the  lower  Oligocene  occur 
rhinoceroses  which  have  the  beginnings  of  a  trans- 
verse pair  of  horns  on  the  nose;  in  the  upper  Oligo- 
cene these  have  increased  to  full  size  and  this  gives 
us  the  exceptional  series  of  paired-horn  rhinoceroses 
referred  to  above.  In  the  lower  Miocene  the  series 
was  represented  by  its  terminal  member,  a  very  small 
animal,  of  which  immense  herds  lived  on  the  Great 
Plains.  Near  Agate,  Nebraska,  is  a  bed,  perhaps  an 
ancient  quicksand,  which  contains  the  bones  of  this 
little  rhinoceros  in  incredible  quantities. 

The  story  of  the  rhinoceroses,  their  development 
and  their  wanderings,  is  a  fascinating  one,  but,  un- 
fortunately, only  this  bald  sketch  can  be  given,  from 
which  the  most  interesting  details  have  been  omitted. 
Their  testimony,  so  far  as  it  goes,  is  just  the  same  as 
in  the  other  genealogical  series,  gradual  differentia- 


114  THE  THEORY  OF  EVOLUTION 

tion,  with  diversification  and  adaptation  to  different 
climates,  food-plants  and  habits  of  life. 

There  remains  one  family  of  mammals  with  which 
it  is  necessary  to  deal  and  that  is  the  camel  tribe. 
This  family  has  two  well-defined  subdivisions,  the 
camels  of  the  Old  World  and  the  llamas,  guanacos, 
etc.,  of  South  America.  For  a  very  long  time,  the 
family  was  entirely  confined  to  North  America  and 
did  not  reach  its  present  homes  until  the  Pliocene 
epoch  of  the  Tertiary  period.  The  skeleton  of  a 
Patagonian  Guanaco  may  be  taken  as  the  starting- 
point  of  our  inquiry.  In  this  animal  the  third  incisor 
and  the  canine  are  retained  in  the  upper  jaw,  all  the 
incisors  and  the  canine  in  the  lower.  The  anterior 
two  grinding  teeth  have  been  lost  and  the  others  are 
moderately  high-crowned.  The  skull  is  broad  and 
capacious  behind,  narrow  and  tapering  in  front. 
The  neck  is  long  and  its  vertebrae  very  curiously 
modified.  The  limbs  are  long  and  slender  and  have 
undergone  nearly  the  same  modifications  as  in  the 
horses;  the  ulna  is  greatly  reduced,  interrupted  in 
the  middle  and  its  separated  portions  are  fused  with 
the  radius.  In  the  hind  leg  the  shaft  of  the  fibula 
has  been  completely  suppressed;  the  upper  end  fuses 
with  the  tibia,  while  the  lower  remains  as  a  small 
separate  bone,  wedged  in  between  the  tibia  and  the 
heel-bone.  The  feet  are  very  long  and  slender,  with 
two  toes  in  each;  the  long  bones  of  the  foot  are  co- 
ossified  to  form  a  "cannon-bone,"  the  very  young 
skeleton  showing  that  this  co-ossification  does  ac- 


EVIDENCE  FROM  PALAEONTOLOGY        115 


FIG.  11.  Four  stages  in  the  evolution  of  the  cameline  skull.  A,  Proty- 
lopus,  upper  Eocene.  B,  Poebrotherium,  lower  Oligocene.  C,  Pro 
camelus,  upper  Miocene.  D,  Guanaco,  Recent. 


116  THE  THEORY  OF  EVOLUTION 

tually  take  place.  The  toes  proper  are  free,  giving 
the  "cloven  hoof,"  but  the  hoofs  are  very  small  and 
the  weight  is  carried  upon  a  soft  thick  pad. 

Were  there  time  enough  to  do  so,  we  might  trace 
the  development  of  this  family  backward,  step  by 
step,  through  all  the  many  stages  between  the  Pleis- 
tocene and  the  upper  Eocene  in  quite  as  unbroken 
sequence  and  in  as  full  detail  as  can  be  done  for  the 
horses.  We  must,  however,  pass  over  all  the  inter- 
mediate steps  and  consider  the  ancestral  camels  of 
the  upper  Eocene.  These  were  very  little  animals, 
hardly  larger  than  a  jack  rabbit,  which  had  the  full 
complement  of  teeth,  44  in  total  number  and  all  with 
very  low  crowns.  The  limbs  and  especially  the  feet, 
are  relatively  short,  the  ulna  is  complete  and  sepa- 
rate, as  is  also  the  fibula;  there  are  four  toes  in  each 
foot,  though  the  lateral  pair  of  the  hind  foot  are  ex- 
tremely slender,  and  there  is  no  co-ossification  to 
form  cannon-bones.  The  hoofs  are  well  developed, 
in  form  like  those  of  an  antelope,  so  that  there 
can  have  been  no  pad.  For  the  present,  the  line 
cannot  be  carried  back  of  the  upper  Eocene,  the 
probable  ancestors  from  the  middle  and  lower  Eocene 
being,  as  yet,  represented  only  by  fragmentary 
specimens. 

In  addition  to  this  main  stem  of  cameline  descent, 
which  resulted  in  the  modern  species,  there  were 
two  short-lived  side  branches  which  should  be  men- 
tioned. One,  ending  in  the  lower  Miocene,  was  the 
series  descriptively  called  "gazelle-camels,"  small 


FIG.  12.  Four  stages  in  the  evolution  of  the  cameline  fore  foot.  A, 
Protylopus,  upper  Eocene.  B,  Poebrotherium,  lower  Oligocene.  C, 
Procamelus,  upper  Miocene.  D,  Guanaco,  Recent. 


118  THE  THEORY  OF  EVOLUTION 

animals  with  very  long  and  slender  legs,  evidently 
swift  runners.  The  other  series,  the  so-called  "gi- 
raffe-camels," terminated  in  the  upper  Miocene; 
these  were  browsers  and  display  an  increasing  stat- 
ure, especially  in  the  length  of  the  neck  and  fore 
limbs.  They  adapted  themselves  to  the  growing 
aridity  of  the  western  plains. 

Finally,  may  be  mentioned  an  example  from  the 
reptiles.  The  pre-eminently  Mesozoic  group  of 
reptiles  known  as  the  Dinosaurs  has  been  made 
quite  familiar  to  the  public  by  our  great  museums, 
notably  the  American  Museum  in  New  York,  the 
National  Museum  in  Washington,  and  the  Carnegie 
Museum  in  Pittsburgh.  In  the  latter  part  of  the 
Cretaceous  period,  North  America  had  a  remarkable 
family  of  Dinosaurs,  harmless,  plant-eating  and 
quadrupedal  monsters.  Though  inoffensive,  they 
were  well  provided  for  defence,  having  three  long 
and  sharp  horns,  one  on  the  nose,  like  that  of  a  rhi- 
noceros, and  one  over  each  eye.  The  neck  was  pro- 
tected by  an  enormous  frill  of  solid  bone,  growing 
out  from  the  skull  and  reaching  back  to  the 
shoulders.  The  family  may  be  traced  far  back 
through  the  Cretaceous,  the  animals  becoming 
smaller  and  the  horns  shorter  as  we  go  back  in 
time,  and  the  neck-shield  becoming  a  mere  outline 
framework.  When  sufficient  well-preserved  material, 
leading  through  long  periods  of  time,  can  be  ob- 
tained, the  result  is  always  suggestive  of  a  gradual 
development. 


EVIDENCE  FROM  PALAEONTOLOGY       115 


E 


FIG.  13.  Skulls  of  Horned  Dinosaurs.  A,  Torosaurus.  B,  Triceratops. 
C,  Monoclonius.  D,  Styracosaurus.  E.  Ceratops.  F,  Brachycera- 
tops.  These  do  not  form  a  single  genealogical  series,  merely  sev- 
eral stages  in  the  development  of  the  skull.  (From  Matthew,  in 
part). 


LECTURE  V 
EVIDENCE  FROM  GEOGRAPHICAL  DISTRIBUTION 

The  geographical  distribution  of  animals  and 
plants  over  the  surface  of  the  globe  was  a  subject  of 
interest  and  importance  even  in  pre-Darwinian  days 
and  great  attention  was  paid  to  it  as  to  so  much 
statistical  information.  Under  the  then  prevailing 
theory  of  special  creation  and  the  immutability  of 
species,  no  explanation  of  the  facts  could  be  expected 
or  even  looked  for;  organisms  were  created  and 
placed  where  we  find  them  and  that,  if  true,  was  an 
ultimate  fact,  of  which  no  explanation  was  possible. 
As  was  pointed  out  in  the  first  lecture,  Darwin  tells 
us  in  his  autobiography  that  it  was  the  facts  of  dis- 
tribution which  he  observed  in  South  America  and, 
above  all,  in  the  Galapagos  Islands,  that  first  turned 
his  thoughts  in  the  channel  of  his  great  work,  be- 
cause these  facts  seemed  to  demand  an  evolutionary 
explanation. 

Every  one  knows  that  different  animals  and  plants 
are  to  be  found  in  different  parts  of  the  world;  the 
polar  bears  and  musk-oxen  occur  in  the  treeless 
wastes  of  the  Arctic  lands,  lions  and  tigers,  elephants 
and  rhinoceroses  and  tall  palms  must  be  sought  in 
warm  countries  and,  on  hasty  consideration,  it  might 
120 


EVIDENCE  FROM  DISTRIBUTION         121 

seem  that  distribution  were  wholly  a  matter  of 
climate.  But  this  explanation  fails  on  closer  exam- 
ination. The  tropical  parts  of  Australia,  Africa  and 
South  America  have  very  similar  climates  and  yet 
it  would  be  difficult  to  select  three  regions  of  the 
earth  which  are  more  radically  distinct  from  the 
zoological  point  of  view.  The  fact  that  a  given 
animal  or  plant  does  not  occur  in  a  particular  region, 
is  no  conclusive  proof  that  it  is  not  fitted  to  live 
there.  The  rabbits  introduced  into  Australia  have 
multiplied  to  such  an  extent  that  they  have  become 
a  very  serious  pest,  as  have  the  European  Gypsy  and 
Brown-tail  Moths  in  Massachusetts.  The  acclima- 
tization of  the  Indian  Mongoose  in  Jamaica  has 
upset  the  whole  zoological  equilibrium  of  the  island, 
while  domesticated  horses  and  cattle,  running  wild, 
grew  into  vast  herds,  which  overspread  the  plains  of 
North  and  South  America.  European  weeds  are 
among  the  most  difficult  and  annoying  enemies 
with  which  the  American  farmer  has  to  contend, 
and  the  American  cactus,  incautiously  brought  into 
South  Africa,  has  become  a  nuisance  and  taken  pos- 
session of  large  areas.  Cactuses  and  the  Agave  (or 
Century  Plant,  also  American)  have  taken  so  kindly 
to  the  Mediterranean  lands,  that  many  artists  have 
painted  them  in  scenes  of  classical  antiquity,  which 
to  the  botanical  geographer  is  amusing.  These 
and  many  similar  facts  sufficiently  show  that  the 
introduction  of  a  new  animal  or  plant  may  be  a  very 
serious  undertaking,  fraught  with  great  possibilities 


122  THE  THEORY  OF  EVOLUTION 

of  mischief  and,  further,  how  excellent  are  the  reasons 
for  the  modern  legislation  which  forbids  the  impor- 
tation of  live  animals  into  the  United  States,  except 
by  permission  of  the  Secretary  of  Agriculture.  No 
one  can  tell  in  advance  what  plague  he  may  be 
bringing  in. 

These  facts  also  prove  definitely  that  animals 
and  plants  may  flourish  in  many  regions  beside  their 
native  homes  and  that  the  absence  of  any  organism 
is  not  to  be  explained  by  assuming  its  unfitness. 
Such  considerations  might  easily  lead  us  to  conclude 
that  the  whole  subject  of  distribution  was  a  meaning- 
less hodge-podge  of  uncorrelated  facts,  which  defied 
explanation  and,  yet,  if  there  be  any  truth  in  the 
theory  of  evolution,  it  must  afford  an  explanation, 
or  else  its  claims  upon  our  acceptance  will  be  very 
greatly  weakened.  The  fundamental  postulate  of 
the  theory  is  the  unbroken  continuity  of  life  from  its 
first  appearance  on  the  earth  and  therefore  the 
present  arrangement  of  living  things  must  be  the 
outcome  of  an  unimaginably  long  series  of  past 
changes,  not  only  those  produced  by  the  evolutionary 
process,  but  also  the  countless  changes  of  climate 
and  geography  which  have  continually  altered  the 
possibility  of  migration  in  this  or  that  direction. 
The  great  extinctions,  which  have  from  time  to  time 
devastated  whole  regions,  have  also  played  a  part  in 
bringing  about  the  present  order  of  things.  At  the 
end  of  the  Pleistocene  epoch,  immediately  preceding 
our  own,  North  America  lost  a  host  of  large  mam- 


EVIDENCE  FROM  DISTRIBUTION         123 

mals,  the  bones  of  which  are  preserved  in  such 
natural  museums  as  the  wonderful  asphalt  pits  near 
Los  Angeles:  horses,  tapirs,  llamas,  elephants  and 
mastodons,  giant  ground-sloths,  huge  wolves,  lions, 
the  terrible  sabre-tooth  tigers,  and  countless  others. 
Here  is  a  zoological  revolution,  which  equally  af- 
fected the  northern  part  of  the  Old  World. 

We  have  very  clear  proof  that  the  form,  size  and 
mutual  connections  of  the  various  continents  have 
undergone  a  long  succession  of  changes.  In  the 
cases  where  two  continents,  now  connected,  were 
formerly  separated  by  the  sea,  the  proof  of  this 
separation  almost  always  remains  in  the  form  of 
marine  deposits,  including  the  fossils  of  marine 
animals.  In  this  manner  the  ancient  disjunction  of 
North  and  South  America,  the  separation  of  the 
Spanish  peninsula  from  Europe,  of  north  Africa 
from  the  remainder  of  the  continent  and  of  India 
from  Asia,  may  be  readily  demonstrated.  On  the 
other  hand,  in  the  case  of  those  lands  which  were 
formerly  connected,  but  now  are  separated  by  the 
sea,  the  direct  proof  is  wanting,  because  the  connec- 
tion is  covered  by  the  sea,  but  soundings  generally 
indicate  it,  for  most  of  the  former  connections  which, 
for  one  reason  or  other,  we  presume  to  have  existed, 
are  marked  out  by  areas  of  shoal  water  in  very  sharp 
contrast  to  the  profound  depths  of  the  oceanic 
abysses.  Such  former  connections  are  indicated 
between  Alaska  and  northeastern  Asia,  Great 
Britain  and  the  continent  of  Europe;  Spain  and 


124  THE  THEORY  OF  EVOLUTION 

Morocco;  Italy,  Sicily,  Malta  and  Tunis;  the  Greek 
islands  and  the  mainland;  Japan,  Formosa,  Borneo, 
Java,  etc.,  and  Asia,  and  many  others.  The  evi- 
dences of  climatic  changes,  though  of  an  entirely 
different  kind,  are  equally  clear. 

In  brief,  the  distribution  of  existing  animals  and 
plants  constitutes  a  series  of  historical  problems, 
the  solution  of  which  is  to  be  sought  in  the  facts  of 
geology  and  palaeontology.  On  evolutionary  prin- 
ciples, we  ought  to  find  that,  the  longer  time  any 
two  regions  have  been  separated,  the  greater  is  the 
difference  between  the  animals  and  plants  of  those 
regions,  and  this  is  precisely  what  we  do  find.  So 
well  established  is  this  principle,  that  we  may  invert 
it  and  use  it  to  distinguish  those  separations  which 
are  earlier  from  those  which  are  later  in  geological 
date,  in  cases  where  direct  geological  evidence  is 
unattainable.  The  separation  of  Great  Britain  from 
the  continent  must  have  taken  place  very  recently, 
geologically  speaking,  because  there  is  no  important 
difference  between  their  animals  or  plants,  while  the 
separation  of  Madagascar  from  Africa  must  have 
been  effected  at  a  much  earlier  time  because  of  the 
radical  differences  between  the  faunas  of  island  and 
continent.  Of  course,  such  instances  cannot  fairly 
be  used  as  arguments  in  favour  of  evolution,  because 
they  assume  the  truth  of  the  thing  to  be  proved, 
except  in  so  far  as  the  evolutionary  explanation  is 
the  only  one  that  will  account  for  the  facts. 

Before  attempting  to  explain  any  of  the  problems 


EVIDENCE  FROM  DISTRIBUTION         125 

of  distribution,  it  will  be  necessary  to  consider  the 
factors  which  determine  distribution  under  present 
conditions  and,  in  order  to  keep  the  whole  great 
subject  within  manageable  limits,  we  must  confine 
our  attention  to  a  few  groups  of  organisms.  For 
our  purposes  by  far  the  most  suitable  examples  are 
those  furnished  by  the  mammals.  All  the  evidence 
at  our  disposal  goes  to  show  that  every  species  orig- 
inated in  some  one  particular  area  and  has  spread 
from  that  starting-point  as  far  as  circumstances 
would  permit.  This  is  called  the  "doctrine  of 
specific  centres."  In  the  case  of  more  comprehen- 
sive groups  also,  there  is  no  reason  to  believe  that, 
among  the  higher  animals,  at  least,  the  same  genus, 
family,  order,  class,  etc.,  ever  arose  twice  independ- 
ently in  disconnected  land  areas.  This  being  the 
case,  the  possession  of  identical,  or  closely  similar, 
animals  by  two  regions  now  separated  is  strong  evi- 
dence that  these  regions  were  once  joined  and  at 
no  very  distant  date.  The  spread  of  new  forms  from 
their  centres  of  origin  may  be  quick  or  slow,  as  cir- 
cumstances are  or  are  not  favourable;  very  rapid 
spread  was  shown  by  the  horses  which,  set  free 
about  1537,  on  the  abandonment  of  the  first  settle- 
ment of  Buenos  Aires,  within  forty-three  years  ap- 
peared on  the  Straits  of  Magellan,  nearly  a  thousand 
miles  away. 

The  nature  of  the  barriers  which  limit  the  spread 
of  any  species  differs  with  the  kinds  of  organisms, 
but  for  land  mammals  the  most  important  are  wide 


126  THE  THEORY  OF  EVOLUTION 

arms  of  the  sea;  no  such  animal  can  pass  the  English 
Channel  or  Bering  Strait.  Next  in  importance  is 
climate  and  especially  temperature,  though  hu- 
midity also  plays  an  effective  part.  As  we  have  seen, 
similarity  of  climate  is,  of  itself,  entirely  powerless 
to  bring  about  similarity  of  animals,  but  as  a  passive 
barrier,  climatic  differences  are  most  effective  in 
limiting  the  spread  of  organisms,  many  of  which  are 
utterly  unable  to  pass  an  isothermal  line.  In  minor 
degree,  large  rivers,  mountain-chains  and  the  other 
great  topographic  features  frequently  act  as  barriers. 
Still  another  kind  of  barrier  is  pre-occupation  by 
some  other  species,  it  may  be  a  competing  species 
which  leaves  no  place  in  the  economy  of  nature 
which  the  newcomer  can  fill,  or  it  may  be  an  enemy 
or  infectious  disease  which  destroys  the  immigrants 
as  fast  as  they  can  arrive.  The  Tse-tse  flies  of  Africa 
close  the  area  of  their  range  against  the  entrance  of 
most  of  the  domestic  animals  by  infecting  them  with 
a  fatal  disease. 

Many  attempts  have  been  made  to  divide  the 
lands  of  the  globe  into  zoological  regions,  which 
shall  express  the  degrees  of  likeness  and  difference 
in  their  animal  inhabitants.  This  cannot  be  satis- 
factorily done  by  using  the  same  scheme  for  all 
kinds  of  organisms,  for  the  geological  dates  of  the 
origin  and  dispersal  of  the  various  types  of  land 
animals  are  so  far  apart  in  time,  that  each  found 
such  very  different  geographical  arrangements,  and 
frequently  also  climatic  conditions,  that  the  possi- 


EVIDENCE  FROM  DISTRIBUTION         127 

bilities  of  migration  had  been  greatly  changed.  For 
the  mammals  a  very  satisfactory  working  arrange- 
ment has  been  gradually  reached,  which  is  well 
justified  by  the  data  of  geology  and  palaeontology. 
This  plan  first  of  all  divides  the  continents  into  two 
groups  of  very  unequal  size,  (1)  Australia  and  its 
adjoining  islands,  Tasmania,  New  Guinea,  etc., 
(2)  the  rest  of  the  world.  This  division  recognizes 
the  fact  that  Australia  is,  zoologically  speaking,  by 
far  the  most  isolated  region  of  the  earth;  aside  from 
the  bats  and  a  few  mice,  its  mammals  are  Mono- 
tremes  and  Marsupials.  Of  the  monotreme  egg- 
laying  mammals  Australia  has  all  existing  species 
and  of  the  marsupials  all  except  the  opossums  of  the 
western  hemisphere,  with  the  addition  in  South 
America  of  one  interesting  genus,  the  survivor  of  a 
group  that  was  abundant  in  the  Tertiary  of  that 
continent. 

Neglecting  Australia,  the  other  continents  likewise 
fall  into  two  very  unequal  groups,  (1)  South  America, 
including  Central  America  and  the  West  Indies, 
and  (2)  the  other  continents.  South  America  is, 
after  Australia,  much  the  most  peculiar  part  of  the 
globe,  a  fact  which  is  explained  by  its  geological 
history.  The  remaining  continents,  North  America, 
Europe,  Asia  and  Africa,  with  great  local  differences 
among  themselves,  have  a  certain  unmistakable 
similarity  in  their  mammals.  The  local  differences 
are  expressed  by  dividing  this  vast  land-area  into  a 
number  of  regions,  with  the  curious  and  unexpected 


128  THE  THEORY  OF  EVOLUTION 

result  that,  except  Europe,  none  of  the  continents 
falls  into  a  single  region.  North  America,  for  ex- 
ample, contains  more  or  less  of  three  regions,  Asia 
and  Africa  each  two.  This  striking  lack  of  coinci- 
dence between  geographical  and  zoological  bound- 
aries is,  at  first  sight,  so  anomalous  as  to  make  the 
whole  scheme  appear  very  doubtful,  but,  here  again, 
past  geographical  and  present  climatic  conditions 
offer  an  adequate  explanation  of  the  anomaly. 

Finer  distinctions  are  expressed  by  dividing  the 
regions  into  subregkms,  provinces,  faunas,  etc.,  but 
it  will  not  be  necessary  to  consider  these  subdivi- 
sions here.  The  following  table,  which  omits  Aus- 
tralia, gives  the  arrangement  of  geographical  realms 
and  regions  which  is  now  very  generally  adopted  by 
students  of  the  mammals,  past  and  present. 

I.  REALM  OF  NEOG.EA. 

1.  Neotropical  Region,  South  and  Central  America;  low- 
kinds  of  Mexico;  southern  tips  of  Florida  and  Lower 
California;  West  Indies. 

1.  Malagasy  Region,  Madagascar. 

2.  Ethiopian  Region,  Africa  south  of  the  Sahara. 

3.  Oriental  Region,  Southern  peninsulas  of  Asia;  Malay 

Archipelago. 

4.  Holarctic  Region,  N.  Africa;  Europe;  Asia  north  of  the 

Oriental  Region,  Japan;  boreal  N.  America. 

5.  Sonoran  Region,  Remainder  of  N.  America. 


n . 

OF 

ABCTOG.EA. 


A  kind  of  distribution  which  has  seemed  very  diffi- 
cult to  explain  is  that  known  as  "discontinuous," 
where  the  areas  inhabited  by  a  given  group,  large 
or  small,  are  separated  by  a  very  wide  intervening 


EVIDENCE  FROM  DISTRIBUTION         129 

space  from  which  it  is  absent.  Two  or  three  ex- 
amples, of  which  history  affords  the  solution,  may 
be  cited.  At  the  present  time  the  family  of  the 
tapirs  is  found  in  the  Oriental  region  and  in  South 
America,  with  almost  half  the  circumference  of  the 
earth  between  them.  In  the  latter  part  of  the  Ter- 
tiary period  the  tapirs  ranged  all  over  the  northern 
hemisphere, — but  became  extinct  in  Europe  before 
the  Pleistocene;  at  the  close  of  the  latter  epoch  they 
vanished  from  eastern  Asia  and  from  North  America, 
persisting  in  southern  Asia  and  in  Central  and  South 
America  to  the  present  day.  We  have  no  means  of 
telling  why  they  became  extinct  in  the  areas  from 
which  they  vanished,  but  the  significant  fact  remains 
that  the  vast  stretch  of  land  between  their  modern 
homes  was  formerly  covered  by  them.  According 
to  the  doctrine  of  special  creation  this  fact  explains 
nothing,  because  the  American  species  are  different 
from  the  Asiatic,  but  this  is  the  almost  inevitable 
result,  if  evolution  be  true.  The  extremes  of  such  a 
wide  ranging  genus  would,  in  the  great  majority  of 
cases,  diverge  to  such  an  extent  that  it  would  be 
necessary  to  refer  them  to  distinct  species. 
/  A  second  case  is  very  similar.  The  camel  family  is 
*  divided  into  two  well-defined  genera,  the  true  camels, 
which  are  native  to  Asia,  and  the  guanacos,  llamas, 
7  etc.,  of  South  America.  This  family,  during  much 
the  longest  part  of  its  history,  was  exclusively  North 
American  in  distribution,  but  in  the  Pliocene  epoch 
true  camels  had  appeared  in  India  and  llamas  had 


\ 


130  THE  THEORY  OF  EVOLUTION 

extended  their  range  to  Argentina.  The  explanation 
of  the  discontinuity  is  the  same  as  in  the  tapirs,  the 
former  occupation  of  the  intervening  lands  by  the 
family  in  question  and  their  eventual  extinction  in 
those  lands. 

iird  case  is  rather  different.  On  the  tops  of  the 
White  Mountains  in  New  Hampshire  grows  an 
assemblage  of  plants,  to  find  which  in  the  lowlands, 
one  must  go  hundreds  of  miles  northward  to  Labra- 
dor. The  probable  reason  for  this  strange  distribu- 
tion is  given  by  the  glacial  phenomena  of  the  Pleis- 
tocene; when  the  ice  had  buried  the  northern  part  of 
the  continent,  Arctic  animals  and  plants  were  driven 
far  to  the  South,  the  walrus  coming  down  to  the 
coast  of  Georgia  and  the  musk-ox  to  Kentucky  and 
Oklahoma.  With  the  amelioration  of  the  climate  and 
retreat  of  the  ice,  the  Arctic  plants  and  animals  once 
more  went  northward  and  ascended  the  mountains, 
as  these  were  freed  from  the  ice.  When  present 
climatic  conditions  had  been  attained,  the  Arctic 
forms  had  disappeared  from  the  foot  of  the  moun- 
tains, but  remained  on  their  cold  summits  as  colonies 
of  the  great  Arctic  invasion.  The  same  thing  hap- 
pened in  Europe,  where  Arctic  plants  still  flourish  in 
the  high  Alps.  Thus,  in  all  cases  where  the  history 
of  the  animals  or  plants  involved  can  be  obtained, 
the  explanation  of  discontinuous  distribution  is  the 
same,  —  former  occupation  of  the  intervening  area. 
The  striking  contrast  between  North  and  South 
America  is  expressed  in  the  scheme  of  classification; 


EVIDENCE  FROM  DISTRIBUTION        131 

North  America  is  a  zoological  patchwork,  consisting 
of  one  entire  region,  the  Sonoran,  a  large  area  of  a 
second,  the  Holarctic,  and  small  portions  of  the 
third,  the  Neotropical,  while,  on  the  other  hand,  the 
southern  continent  is  all  included  in  the  Neotropical 
region.  This  contrast  is  explained  by  the  different 
geological  history  of  the  two  continents.  During  the 
earlier  and  longer  moiety  of  the  Tertiary  period 
North  and  South  America  were  separated  by  a  broad 
sea  which  swept  over  the  Isthmus  of  Panama  and 
most  of  Central  America  and  throughout  this  time  of 
separation  the  northern  and  southern  continents 
developed  faunas  which  were  most  radically  different. 
The  student  who  is  familiar  with  the  Tertiary  mam- 
mals of  the  northern  hemisphere,  American,  Euro- 
pean and  Asiatic,  finds  himself  in  a  new  and  strange 
world,  when  he  takes  up  those  of  South  America. 
On  the  other  hand,  North  America  was  repeatedly 
joined  to  the  Old  World  and  as  often  severed  from  it, 
chiefly  by  way  of  Alaska  and  northeastern  Asia,  but 
probably  also  by  a  land-bridge  from  Greenland  to 
Scandinavia.  It  is  almost  always  easy  to  distinguish 
the  times  of  separation  from  those  of  union,  for  the 
former  are  marked  by  a  rapid  divergence  from  the 
Old  World  forms,  while  the  latter,  because  of  the 
intermigration  of  mammals,  always  possess  a  certain 
number  of  families  and  genera  which  are  common  to 
both  hemispheres.  The  latest  of  these  invasions 
from  Asia  took  place  in  the  Pleistocene  and  brought 
in  a  crowd  of  Old  World  mammals,  bears  and  wol- 


132  THE  THEORY  OF  EVOLUTION 

verenes,  sheep,  antelopes,  bison,  the  wapiti  (erro- 
neously called  "elk")  the  moose,  the  caribous  and 
many  others,  and  these  continue  to  inhabit  the 
northern  half  of  the  continent  and  have  given  to  it 
that  Old  World  stamp  which  is  recognized  by  in- 
cluding this  boreal  area  in  the  great  Holarctic  region. 
Many  species,  which  are  now  extinct,  accompanied 
this  Pleistocene  invasion,  the  most  conspicuous  of 
which  was  the  Siberian  mammoth,  or  hairy  elephant, 
which  ranged  from  the  south  of  France  to  Kamchatka 
and,  on  the  American  side,  from  Alaska  to  New 
England.  Systematic  naturalists  still  debate  the 
question  as  to  whether  these  immigrants  have  suffi- 
ciently changed  since  their  arrival  in  America  to 
deserve  to  rank  as  species  distinct  from  their  Old 
World  relatives.  Whether  the  American  and  Euro- 
pean bison,  the  Scandinavian  and  American  moose, 
the  wapiti  of  America,  that  of  Asia  and  the  European 
red  stag,  the  caribou  and  the  reindeer,  are  valid 
species  or  merely  geographical  races  of  the  same 
species,  need  not  be  discussed  here.  Whichever  view 
we  take,  the  Old  World  character  of  the  northern 
half  of  our  continent  is  not  affected. 

Between  this  "boreal  zone"  and  the  Sonoran 
region,  there  is  a  "transition  zone"  where  the  two 
faunas  mingle.  The  Sonoran  region,  which,  roughly 
speaking,  includes  the  United  States,  except  the 
high  mountain  ranges,  which  carry  the  northern 
fauna  far  to  the  South,  and  the  plateau  of  Mexico, 
is  the  characteristically  North  American  region.  Its 


EVIDENCE  FROM  DISTRIBUTION         133 

deer  are  all  different  from  the  northern  genera  of  the 
family,  which  came  in  during  the  Pleistocene;  they 
too  were  ultimately  derived  from  the  Old  World,  but 
at  a  very  much  earlier  date,  probably  as  far  back  as 
the  lower  Miocene  epoch.  This  Sonoran  fauna  was 
greatly  impoverished  by  the  Pleistocene  extinctions, 
which  put  an  end  to  so  many  of  the  ancient  American 
groups,  but  it  retains  a  large  number  of  characteristic 
forms,  chiefly  rodents  and  carnivores,  or  beasts  of  prey. 

Besides  the  immigration  from  the  eastern  hemi- 
sphere, there  were  successive  waves  of  invaders  from 
the  Neotropical  region;  the  earliest  one  of  which  we 
have  found  a  trace  arrived  in  the  Miocene  and  the 
latest  in  the  Pleistocene,  when  North  America  had 
several  characteristic  Neotropical  forms,  the  most 
striking  of  which  were  the  giant  ground  sloths,  and 
the  huge,  armoured,  armadillo-like  glyptodonts,  both 
now  extinct.  Of  this  southern  invasion  little  now 
remains;  the  Canada  porcupine  is  certainly  one 
survivor  from  the  invasion  and  probably,  though  not 
certainly,  the  opossum  is  another.  The  influence  of 
climate  upon  present  distribution  is  very  well  shown 
in  Mexico;  the  high  table-land,  with  its  moderate  and 
relatively  dry  climate,  is  Sonoran,  while  the  hot,  low- 
lying  coast-lands  are  Neotropical. 

When  the  existing  mammals  of  South  America  are 
reviewed,  they  are  plainly  separable  into  two  cat- 
egories. In  the  first  are  included  those  which  are 
evidently  indigenous  and  are,  for  the  most  part,  very 
unlike  those  of  any  other  continent.  The  monkeys 


134  THE  THEORY  OF  EVOLUTION 

and  marmosets  are  but  remotely  related  to  the  mon- 
keys and  apes  of  the  Old  World;  the  horde  of  mos1 
curious  creatures  which  we  call  ant-eaters,  sloths 
and  armadillos,  are  altogether  peculiar  to  the  Neo- 
tropical region;  the  extremely  large  and  varied  assem- 
blage of  porcupine-like  rodents  are  either  peculiai 
or,  in  a  few  cases,  related  to  those  of  Africa.  The 
many  species  of  opossums  are  related,  though  no1 
very  closely,  to  the  marsupials  of  Australia.  Aus- 
tralia excepted,  no  other  continent  has  so  manj 
different  kinds  of  mammals  peculiar  to  itself  as 
South  America,  and  the  birds  are  almost  equally 
characteristic.  The  second  category  contains  those 
mammals  which  are  nearly  related,  often  belonging 
to  the  same  genera,  to  those  which  now  live,  01 
formerly  lived,  as  shown  by  the  fossils,  in  North 
America.  This  section  includes  all  the  beasts  oi 
prey,  the  wolves,  large  and  small  cats,  weasels, 
otters,  skunks,  raccoons;  all  the  hoofed  animals,  the 
tapirs,  peccaries  (native  swine)  llamas,  deer  (all  oi 
which  belong  to  the  Sonoran  group),  the  rats  and 
mice,  squirrels  and  rabbits. 

This  twofold  division  immediately  suggests  that 
the  mammals  belonging  to  the  second  category  are 
immigrants  from  the  North  and  the  suggestion  is 
very  strongly  confirmed  by  the  testimony  of  the 
fossils.  The  geology  of  the  Isthmian  region  proves 
that  South  America  was  formerly  cut  off  from  the 
northern  continent  and  if  we  examine  the  fossil 
remains  of  the  mammals  which  lived  there  during 


EVIDENCE  FROM  DISTRIBUTION        135 

that  time  of  isolation,  we  find,  as  already  stated, 
that  they  are  totally  distinct  from  their  contem- 
poraries of  the  north.  The  rocks  of  Patagonia  and 
Argentina  have  proved  to  be  quite  as  marvellous  a 
museum  of  vanished  mammalian  life  as  those  of  the 
western  United  States  and  they  tell  the  story  of 
origins  and  migrations  with  the  greatest  clearness. 
The  older  Miocene  rocks  of  Patagonia  have  already 
yielded  to  the  zealous  collector  a  great  host  of 
beautifully  preserved  mammalian  remains,  among 
which  cannot  be  found  a  single  one  that  was  ancestral 
to  any  of  the  mammals  of  the  second  category, 
which,  on  other  grounds,  we  concluded  to  have  been 
derived  from  the  north.  On  the  other  hand,  we  find 
the  ancestors  of  nearly  all  the  peculiarly  South 
American  forms,  except  those  which,  there  is  reason 
to  believe,  never  ranged  so  far  south  as  Patagonia. 
There  was  a  great  abundance  of  hoofed  animals, 
but  they  all  belonged  to  orders  which  have  long  been 
extinct,  and  there  were  many  beasts  of  prey,  large 
and  small,  but  they  were  not  true  Carnivora,  but 
rather  resembled  the  predaceous  marsupials  of 
Australia.  South  America  suffered  even  more  from 
the  Pleistocene  extinctions  than  did  North  America 
and  lost  not  only  many  indigenous  orders,  such  as 
the  ground  sloths  and  glyptodonts,  of  which  there 
was  an  incredible  variety,  and  the  native  hoofed 
animals,  but  also  several  of  the  immigrants  as  well, 
such  as  the  mastodons,  horses  and  antelopes,  the 
great  sabre-tooth  tigers  and  short-faced  bears. 


136  THE  THEORY  OF  EVOLUTION 

There  is  a  remarkable  contrast  between  the  fates 
that  befell  the  northern  invaders  of  the  Neotropical 
region,  and  the  southern  invaders  of  the  Sonoran 
region.  The  former,  in  most  cases,  conquered  a 
permanent  footing  in  South  America  and  their  de- 
scendants, more  or  less  modified,  are  living  there 
now,  while  the  latter  could  not  maintain  themselves 
and  only  one,  or  at  most  two,  has  survived  to  the 
present  time.  The  reason  for  this  very  striking 
difference  is  probably  to  be  found  in  the  fact  that 
the  northern  animals  were  of  higher  type  and  more 
advanced  grade  than  the  southern.  North  America 
was  a  part  of  the  vast  land-area  of  Arctogsea,  a  con- 
dition much  more  favourable  to  the  development  of 
the  higher  forms  than  was  the  insular  state  of  South 
America.  Such  a  conclusion  might  seem  to  be  con- 
tradicted by  Central  America,  which  is  Neotropical 
not  Sonoran,  but  here  the  dominant  factor  is  ob- 
viously climatic,  as  is  shown  by  the  conditions  in 
Mexico.  The  native  and  immigrant  mammals  of 
North  America  are  Arctic  and  Temperate  and  most 
of  them  are  not  adapted  to  life  in  the  Tropics,  yet 
in  spite  of  this,  many  have  permanently  established 
themselves  in  South  America,  while  the  southern 
immigrants  failed  to  keep  their  footing  in  the  north. 

The  zoological  realms  and  regions  which  were  estab- 
lished only  to  express  the  present  order  of  things, 
thus  prove  themselves  to  be  an  expression  of  past 
changes.  Even  were  nothing  known  of  the  geology 
and  palseontology  of  the  western  hemisphere,  but 


EVIDENCE  FROM  DISTRIBUTION         137 

merely  the  existing  distribution  of  its  mammals,  it 
would  be  entirely  possible  to  deduce  from  that  dis- 
tribution the  main  outlines  of  the  history  which 
geology  reveals.  The  geological  dates  could  not  be 
so  definitely  fixed,  but  the  principal  events  and  the 
order  in  which  they  occurred  could  be  made  out, 
assuming  the  truth  of  the  evolutionary  theory.  The 
very  fact  that  such  an  assumption  is  necessary,  if 
the  deductions  are  to  harmonize  with  the  facts  re- 
vealed by  geology  and  palaeontology,  is  strong  evi- 
dence that  the  assumption  is  true.  On  the  theory 
of  special  creation  and  the  immutability  of  species, 
the  facts  are  meaningless.  We  find  that  a  certain 
proportion  of  South  American  forms  is  extremely 
peculiar,  having  no  close  allies  in  any  other  conti- 
nent, and  a  certain  proportion  is  of  mammals  clearly 
related  to  those  which  dwell  in  Arctogsea.  But  the 
latter  all  belong  to  peculiar  species  and  many  of 
them  to  peculiar  genera,  which,  on  this  theory  means 
that  they  were  separately  created  and  therefore  that 
their  relationships  with  North  American  and  other 
Arctogaean  forms  are  purely  ideal.  It  is  highly  sig- 
nificant that  the  history,  so  far  as  it  has  been  recov- 
ered and  deciphered,  is  seen  to  lead  gradually  through 
a  long  series  of  changes  to  the  present  order,  which 
is  so  plainly  the  inevitable  outcome  of  those  changes, 
provided  only  that  evolution  be  accepted. 

It  may  well  be  asked  why  no  such  interchange  of 
mammals  between  the  Americas  is  going  on  now,  but 
the  question  cannot  be  definitely  answered.  It  is 


138  THE  THEORY  OF  EVOLUTION 

probable,  however,  that  a  state  of  equilibrium  has 
been  reached  after  the  readjustments  made  neces- 
sary by  the  last  geographical  and  climatic  changes. 
On  the  other  hand,  a  slow  movement  may  actually 
be  in  progress,  as  is  suggested  by  the  geographical 
races  of  the  Virginia  Deer  from  Mexico  to  Guiana. 
Central  America  contains  a  considerable  number  of 
Sonoran  mammals,  which  have  not  yet  passed  the 
Isthmus,  but  might  perhaps  eventually  have  done 
so,  had  not  the  construction  of  the  canal  intervened. 

The  elevation  of  Central  America  and  the  Isthmus 
cut  off  the  waters  of  the  Caribbean  Sea  from  those 
of  the  Pacific  and  the  separation  led  to  divergent 
development  of  marine  organisms  in  the  divided 
waters.  A  careful  comparative  study  has  been  made 
of  the  sea-fishes  on  the  two  sides  of  the  Isthmus,  with 
the  result  that  nearly  all  the  species  are  different, 
while  the  genera  and  families  are,  for  the  most  part, 
the  same.  The  amount  of  difference  is  what  might 
have  been  expected  d  priori  on  the  evolutionary 
hypothesis  and,  indeed,  the  ichthyologists  had  ap- 
proximated the  true  date  of  the  separation  before 
the  geological  evidence  had  been  found. 

There  is  no  time  to  go  into  the  highly  complex 
history  of  the  vast  aggregation  of  land  in  the  Old 
World,  further  than  to  point  out  that  as  late  as  the 
Pleistocene  epoch,  when  primitive  men  were  already 
living  in  Europe,  that  continent  underwent  a  re- 
markable series  of  climatic  and  geographical  changes. 
At  the  time  of  greatest  elevation,  the  Adriatic  and 


EVIDENCE  FROM  DISTRIBUTION         139 

yEgean  Seas  were  land;  the  junction  of  Italy  and 
Spain  with  Africa  cut  the  Mediterranean  into  two 
land-locked  basins;  the  North  Sea  was  land  and  the 
British  Islands  were  joined  to  each  other  and  to  the 
continent.  This,  taken  in  connection  with  the 
barrier  made  by  the  Desert  of  Sahara,  explains  why 
North  Africa  belongs  now  to  the  great  Holarctic 
region  and  not  to  the  Ethiopian,  which  covers  the 
continent  south  of  the  desert. 

I  do  not  wish  to  give  the  impression  that  all  the 
problems  of  distribution  have  been  solved  and  all  the 
difficulties  removed,  for  such  an  impression  would 
be  far  from  the  truth.  What  can  fairly  be  said  is 
that  the  main  outline  has  been  satisfactorily  ex- 
plained on  the  evolutionary  theory  and  no  other 
theory  even  pretends  to  account  for  the  facts.  Just 
in  proportion  to  the  completeness  with  which  the 
past  history  of  any  mammalian  group  is  known,  is 
it  easy  to  account  for  its  present  distribution.  In 
the  same  way,  the  more  fully  the  geographical  and 
biological  history  of  any  continent  has  been  ascer- 
tained, the  more  readily  can  the  arrangement  and 
relationships  of  its  animals  and  plants  be  under- 
stood. It  is  largely  because  the  history  is  incom- 
plete, that  difficulties  still  remain. 

It  is  often  very  puzzling  to  explain  why  groups 
that  range  together  in  the  region  of  their  origin 
should  behave  so  differently  when  new  land-con- 
nections are  established  and  new  regions  invaded; 
of  this  there  are  very  many  instances.  The  mas- 


140  THE  THEORY  OF  EVOLUTION 

todons  and  the  true  elephants  lived  together  all  over 
North  America,  but  only  the  mastodons  entered 
South  America.  A  possible  explanation  of  this  may 
be  that  the  species  of  elephants  were  cold-country 
animals  which  had  come  in  from  the  north  and  were 
unable  to  cross  the  Tropics,  but  no  such  considera- 
tions will  apply  to  the  Siberian  rhinoceros  and  mam- 
moth. Both  were  well  protected  against  the  cold 
and  both  extended  their  range  in  the  Pleistocene 
from  southwestern  France  to  Kamchatka;  the  mam- 
moth passed  over  to  Alaska  and  crossed  North 
America  to  New  England,  but  the  rhinoceros  has 
never  been  found  on  the  American  side.  Deer  mi- 
grated repeatedly  from  the  Old  World  to  the  New, 
but  the  giraffes,  which  were  once  very  numerous  and 
varied  in  all  the  Mediterranean  lands,  did  not  accom- 
pany them.  Wolves,  foxes,  cats,  bears  migrated  back 
and  forth,  but  America  never  had  any  hyenas  or 
civets  and  the  raccoon  family  has  always  been  Amer- 
ican, though  not  quite  exclusively  so.  The  large, 
spotted  cats,  called  jaguars,  do  not  extend  north  of 
Texas  or  south  of  the  Argentine  Republic,  yet  the 
very  nearly  allied  pumas  range  from  Pennsylvania 
far  into  Patagonia.  These  are  closely  allied  species 
and  yet  their  adaptability  to  climate  is  very  differ- 
ent. Deer  and  antelopes  both  invaded  South  Amer- 
ica, presumably  together,  yet  the  deer  maintained 
themselves,  while  the  antelopes  failed  to  do  so. 
Then,  too,  there  is  much  mystery  about  the  extinc- 
tion of  species  and  larger  groups;  hi  a  general  way 


EVIDENCE  FROM  DISTRIBUTION         141 

We  can  understand  why  this  should  have  happened, 
but,  in  a  given  case,  it  is  not  possible  to  do  more 
than  conjecture  the  reasons. 

Another  series  of  observations  and  deductions, 
which  depend  for  their  cogency  little  or  not  at  all 
upon  the  data  of  geology  and  palaeontology,  is  that 
afforded  by  the  phenomena  of  the  distribution  of 
animals  and  plants  on  islands.  Darwin's  attention 
was  strongly  arrested  by  what  he  observed  in  the 
Galapagos,  and  A.  R.  Wallace,  who,  it  will  be  remem- 
bered, independently  reached  the  same  conclusions 
as  Darwin,  was  led  to  his  results  largely  by  his  studies 
in  the  East  Indian  islands.  To  the  end  of  his  life 
Wallace  was  fascinated  by  this  topic  and  his  two  de- 
lightful books,  "The  Malay  Archipelago"  and  "Is- 
land Life,"  testify  to  his  unfailing  interest  in  its 
problems.  The  problems  of  insular  life  are  much 
simpler  and  more  comprehensible  than  those  of 
continental  distribution;  they  are  like  experiments 
performed  upon  a  small  scale  and  with  relatively 
few  factors  involved. 

From  the  point  of  view  of  animal  and  plant  dis- 
tribution there  are  two  clearly  distinguished  classes 
of  islands:  (1)  Continental  and  (2)  Oceanic.  Con- 
tinental islands  are,  as  their  name  implies,  detached 
fragments  of  a  continent,  with  which  they  were  at 
one  time  joined;  their  geological  structure  is  that  of 
the  continent  and  usually  they  are  not  far  from  the 
parent  continent  and  are  surrounded  by  shoal  water. 
It  should  be  explained  that  the  line  between  shallow 


142  THE  THEORY  OF  EVOLUTION 

and  deep  water  is  commonly  taken  as  the  100-fathom 
depth;  this  is  not  an  arbitrary  selection,  but  the 
reasons  for  the  choice  cannot  be  discussed  here. 
Several  of  the  best  known  of  the  continental  islands, 
such  as  Great  Britain  and  Ireland,  Borneo,  Sumatra, 
Java,  Madagascar,  have  been  mentioned  in  this  lec- 
ture. Their  animals  and  plants  are  those  of  a  con- 
tinent and  the  degree  of  difference  from  the  neigh- 
bouring mainland  is  an  indication  of  the  date  of 
disjunction.  In  a  few  cases  continental  islands  rise 
from  great  depths  of  water,  as  is  true  of  the  Greater 
Antilles,  and  some  are  far  from  any  land,  like  New 
Zealand  and  the  Seychelles.  Oceanic  islands,  on 
the  other  hand,  are  all  either  volcanic  and  have  been 
built  up  from  the  sea-floor  by  the  material  ejected 
in  the  eruptions  of  submarine  volcanoes,  or  they  are 
coral  reefs.  As  a  rule  they  rise  from  the  profoundest 
depths  of  the  sea  and  are  far  removed  from  any  land 
and  they  appear  never  to  have  formed  part  of  any 
continent.  Examples  of  this  class  are  the  Azores, 
Madeira,  the  Canaries,  Cape  Verdes,  Hawaiian 
Islands  and  the  thousands  which  dot  the  South 
Pacific. 

The  two  classes  of  islands  differ  as  radically  in 
their  animals  as  they  do  in  structure.  The  con- 
tinental islands  may  and  frequently  do  contain 
everything  that  an  equal  area  of  the  parent  main- 
land had  at  the  time  of  separation,  though  in  this 
respect  there  is  naturally  a  difference  between  large 
and  small  islands.  Sometimes  it  is  difficult  to  un- 


EVIDENCE  FROM  DISTRIBUTION         143 

derstand  why  a  large  continental  island  should  be 
so  poor  in  mammals.  The  contrast  between  the 
large  islands  of  the  East  and  West  Indies  is  very  re- 
markable; Borneo,  Sumatra  and  Java  are  extremely 
rich  in  mammals,  while  Cuba,  Haiti  and  Porto  Rico 
have  very  few.  The  oceanic  islands  have  peculiarly 
limited  faunas;  on  the  theory  of  special  creation,  we 
should  expect  to  find  in  them  as  large  a  variety  of 
animals  as  their  area,  their  vegetation  and  their 
climate  would  enable  them  to  support.  On  the 
theory  of  evolution,  they  must  have  received  only 
such  plants  and  animals  as  could  be  carried  to  them 
by  the  wind,  or  currents  of  the  sea,  carried  on  drift- 
wood or  floating  trees,  or  transported  by  birds. 
When  an  oceanic  island  is  very  far  from  the  nearest 
land,  the  accession  of  a  new  animal  or  plant  must  be 
a  very  rare  event. 

That  islands  are  actually,  not  merely  hypothetic- 
ally,  stocked  in  this  manner  is  clearly  shown  by  the 
case  of  Krakatoa,  a  volcanic  island  not  far  from 
Java.  The  great  eruption  of  1883  blew  a  great  part 
of  the  island  out  of  existence  and  buried  the  re- 
mainder so  deeply  under  volcanic  debris,  that  every 
living  thing  perished.  In  a  surprisingly  short  time 
the  island  was  re-vegetated  and  abundant  insect 
life  arrived,  all  borne  by  the  winds  and  the  sea  from 
Java  and  Sumatra.  Professor  Selenka  writes  of  a 
visit  to  Krakatoa:  "Under  the  shade  of  a  Casuarina, 
amidst  cocoa-nut  palms  and  thickets  as  high  as  my 
head,  I  found  here,  to  my  astonishment,  an  active 


144  THE  THEORY  OF  EVOLUTION 

animal  life  of  spiders,  flies,  bugs,  beetles,  butterflies; 
even  lizards  more  than  two  feet  long  animated  the 
peaceful  picture."  In  this  instance,  however,  the 
distance  from  land  was  not  very  great,  or  else  the 
restocking  would  have  required  a  very  much  longer 
time. 

The  oceanic  islands  have  no  land  mammals,  except 
bats,  which  like  birds  are  often  carried  immense  dis- 
tances by  strong  gales.  That  this  lack  of  mammals 
is  due  to  the  impossibility  of  reaching  the  islands 
and  not  to  any  unfitness  to  support  the  mammals  is 
made  clear  by  the  artificially  introduced  rabbits, 
rats,  mice,  pigs,  goats  and  cattle,  which  have,  on 
many  such  islands,  thriven  and  multiplied  exceed- 
ingly. They  also  lack  Amphibia,  for  frogs  and 
toads,  newts  and  salamanders,  their  eggs  and  larvae, 
cannot  long  endure  immersion  in  sea- water;  real 
fresh-water  fishes,  crayfishes  and  shells,  as  distin- 
guished from  those  which  readily  enter  rivers  from 
the  sea,  are  likewise  wanting.  Birds  and  insects 
are  brought  by  the  winds,  reptiles,  especially  lizards, 
and  snails  are  carried  on  driftwood,  and  it  is  only 
such  groups  that  occur  in  the  oceanic  islands. 

The  observations  which  Darwin  made  in  the 
Galapagos,  a  volcanic  group  about  500  miles  from 
the  coast  of  Ecuador,  will  serve  as  a  type  of  what 
may  be  learned  from  the  life  of  oceanic  islands. 
Leaving  out  of  account  the  sea-birds,  which  can 
traverse  the  widest  expanses  of  sea,  the  birds,  rep- 
tiles and  plants  of  the  Galapagos  for  the  most  part 


EVIDENCE  FROM  DISTRIBUTION         145 

belong  to  peculiar  species,  but  all  are  of  South 
American  type  and  have  their  nearest  relatives  in 
that  continent.  Each  of  the  larger  islands  of  the 
group  has  species  peculiar  to  itself,  but  almost  all 
the  species  distributed  on  the  various  islands  belong 
to  the  same  or  nearly  allied  genera,  and  thus  to- 
gether form  a  homogeneous  and  closed  group.  The 
explanation  which  Darwin  gave  to  the  phenomena 
still  remains  the  most  satisfactory  that  has  been 
offered.  The  distance  from  the  mainland  is  so  great, 
that  for  any  bird  or  lizard  to  reach  the  islands  must 
always  have  been  a  rare  event,  but  a  few  species  did 
arrive  and  were  thus  practically  cut  off  from  their 
relatives  of  the  continent.  Communication  be- 
tween the  different  islands  is  likewise  rare  and  so 
each  became  the  seat  of  a  little  colony  which  gradu- 
ally changed  from  its  former  condition  to  form  races, 
varieties  and  eventually  species.  Thus,  the  evolu- 
tionary theory  accounts  for  the  facts  in  a  simple 
and  adequate  manner,  while  the  rival  theory  offers 
no  explanation.  The  Cape  Verde  Islands,  off  the 
west  coast  of  Africa  are  a  volcanic  group  quite  like 
the  Galapagos,  but  their  animals  are  of  African 
type  modified  through  isolation  and  the  Galapagos 
kind  of  distribution  is  not  the  exception,  but  the 
rule  under  similar  conditions. 

Whether  new  species,  say  of  birds,  will  arise  in 
an  oceanic  island,  depends  on  the  frequency  with 
which  mainland  birds  can  visit  it  and  thus  by  con- 
stant crossing  maintain  the  continental  character- 


146  THE  THEORY  OF  EVOLUTION 

istics.  Such  islands  as  Bermuda  and  Madeira  are 
annually  visited  by  birds  from  the  adjoining  con- 
tinents and  they  have  hardly  any  peculiar  forms, 
while  St.  Helena  and  the  Hawaiian  group  have 
almost  all  their  birds  and  reptiles  peculiar  to  them- 
selves and  so  far  changed,  that  it  is  difficult  to  make 
out  their  geographical  relationships.  If  species  are 
immutable,  why  should  this  factor  of  inaccessibility 
have  any  influence  upon  the  peculiarity  of  the  species 
found  in  a  given  island?  Another  curious  series  of 
facts  is  displayed  by  the  family  of  birds  known  as 
the  Rails,  which  are  distributed  all  over  the  world 
except  the  polar  regions.  The  family  includes  fifty- 
five  genera,  twenty-five  of  which  inhabit  islands 
and  of  these  ten  have  lost  the  power  of  flight.  A 
genus,  with  only  one  or  two  species,  will  be  confined 
to  a  single  island,  while  the  genera  of  the  continents 
have  more  numerous  species  and  all  of  them  can  fly. 
It  is  an  obvious  inference  that  the  loss  of  flight  and 
the  separation  as  distinct  genus  and  species  must 
have  taken  place  after  the  original  progenitors  had 
reached  a  given  oceanic  island,  for  the  birds  could 
not  have  crossed  wide  seas  by  swimming,  nor  could 
the  flightless  ones  have  arrived  in  any  other  way. 

"If  we  briefly  resume  these  surveys  of  island 
faunas,  the  result  for  our  problem  is  as  follows: 
The  islands  must  have  received  their  animals  in 
some  fashion  from  the  mainland,  the  continental 
islands  when  they  were  a  part  of  it,  the  oceanic  not 
till  after  their  appearance  above  the  surface  of  the 


EVIDENCE  FROM  DISTRIBUTION         147 

sea.  This  may  be  directly  proved  for  young  oceanic 
islands.  Now,  since  we  have  all  stages  from  the 
youngest  to  the  oldest  islands  and  see  how  the 
degree  of  specialization  of  the  animals  increases  in 
just  such  a  graduated  way,  and  how  their  number 
and  kind  are  different  in  correspondence  with  the 
distance  of  the  island  and  the  animals'  power  of 
dispersal  and  how  they  everywhere  display  a  rela- 
tionship with  those  of  the  nearest  lands,  the  conclu- 
sion is  cogent  that  even  on  the  most  ancient  islands 
the  animals  were  not  created,  but  immigrated  after 
the  islands  had  been  formed.  If  this  be  admitted, 
the  further  conclusion  cannot  be  escaped,  that  the 
greater  or  less  difference  of  the  insular  forms  from 
those  of  their  former  home  must  be  ascribed  to  a 
mutability  and  capacity  of  change  on  the  part  of 
the  animals."  l 

As  in  the  case  of  all  the  topics  dealt  with  in  the 
preceding  lectures,  I  cannot  but  lament  the  inade- 
quacy of  treatment  which  is  made  necessary  by  the 
demands  of  brevity.  Nobody  would  pretend  for  a 
moment  that  we  understand  all  the  phenomena  of 
distribution  and  can  account  for  every  seemingly 
anomalous  fact,  or  that  all  difficulties  have  been 
removed.  We  are  attempting  to  reconstruct  a  long 
history  of  past  changes,  and  under  such  circum- 
stances, partial  success  is  all  that  can  be  hoped  for. 
But  if,  in  looking  over  the  vast  array  of  data  that 
have  been  gathered  illustrating  distribution  and 

1  A.  Brauer.  iu  Die  Abstammungslehre,  pp.  161-2. 


148  THE  THEORY  OF  EVOLUTION 

comparing  these  with  the  results  of  geology  and 
palaeontology  which  bear  upon  the  same  problems, 
we  ask  whether  the  theory  of  evolution  or  that  of 
special  creation  offers  the  most  satisfactory  explana- 
tion, an  unprejudiced  judge  would  not  long  hesitate 
in  giving  his  decision  in  favour  of  evolution. 


LECTURE  VI 
EVIDENCE  FROM  EXPERIMENT— CONCLUSION 

Experiment  is  the  method  used  in  physics  and 
chemistry,  to  determine  the  laws  of  matter  and  force; 
such  experiments  can  be  repeated  at  will  and  any 
competent  person  can  obtain  the  same  results.  It 
is  highly  desirable  that  the  experimental  method 
should  be  applied  in  biology,  because  of  the  exacti- 
tude which  can  thus  be  secured,  affording  an  in- 
valuable complement  to  the  results  of  observation. 
Darwin  always  insisted  upon  the  indispensable  char- 
acter of  experiments  and  performed  a  great  many 
himself.  One  might  have  anticipated  that  the  prob- 
lems of  evolution  could  be  thus  attacked  with  suc- 
cess and  subjected  to  the  tests  of  a  precise  analysis. 
But  such  hopes  have  left  out  of  account  the  difficul- 
ties inherent  in  the  application  of  the  method;  human 
life  is  very  brief  and  experiments,  to  be  conclu- 
sive, must  often  be  continued  without  interruption 
through  a  very  long  series  of  generations.  Then, 
too,  it  is  impracticable  to  make  conditions  in  the 
laboratory  sufficiently  like  those  hi  the  open  to  give 
definite  answers  to  many  questions. 

In  a  sense,  the  long  continued  improvement  of 
domesticated  animals  and  plants  by  careful  breed- 

149 


150  THE  THEORY  OF  EVOLUTION 

ing  and  selection  has  been  by  a  great  series  of  experi- 
ments, but,  unfortunately,  the  conditions  under 
which  these  were  carried  out  are  too  inexact  and  the 
records,  which  are  usually  lacking  altogether,  are, 
even  when  preserved,  too  vague  to  be  of  much  service 
in  a  modern  investigation.  Domestication,  as  was 
pointed  out  at  some  length  in  a  previous  lecture 
(p.  36),  shows  how  surprisingly  animals  and  plants 
may  be  changed  by  human  agency,  but  tells  us  little 
of  what  we  are  now  seeking  to  learn. 

The  changes  which  are  experimentally  produced 
must  be  transmissible  to  the  offspring,  if  they  are 
to  have  any  evolutionary  significance.  Plants,  in 
particular,  are  very  susceptible  to  extensive  modifica- 
tion through  external  influences,  such  as  the  chemical 
constitution  of  the  soil,  heat,  light,  moisture,  eleva- 
tion above  sea-level,  and  the  like,  and  these  modifica- 
tions will  continue  as  long  as  the  external  conditions 
remain  the  same,  but  disappear  when  the  plants 
are  once  more  placed  under  the  original  circum- 
stances. The  French  botanist  Bouvier  took  shoots 
or  layers  from  several  kinds  of  common  plants  and 
set  out,  from  the  same  individual,  some  shoots  in 
the  lowlands,  others  high  up  in  the  mountains.  The 
differences  in  the  results  were  remarkable;  the  moun- 
tain plants  had  more  vigorous  roots,  but  the  part 
above  ground  was  much  smaller,  with  more  delicate 
stem  and  leaves;  the  flowers  were  fewer  in  number, 
but  individually  larger  and  more  intensely  coloured. 
When  taken  back  to  the  lowlands,  the  plants  re- 


EVIDENCE  FROM  EXPERIMENT          151 

turned  to  their  original  type.  The  same  species  of 
Ranunculus  may  be  grown  on  land  or  in  the  water, 
with  totally  different  forms  of  leaf  in  the  two  media. 
The  land  individuals  have  broad,  tripartite  leaves, 
with  serrate  edges,  while  in  the  water  plants  the 
leaves  are  subdivided  into  long,  thread-like  fila- 
ments and  the  whole  appearance  of  the  plant  is 
that  of  a  different  species.  "In  all  these  cases,  the 
plant  has  not  inherited  a  definite  form  from  its 
parents,  but,  within  the  limits  of  the  hereditary 
fundamental  plan  of  structure,  it  has  received  cer- 
tain possibilities  of  development,  which  did  not 
appear  in  the  parents.  Which  of  these  possibilities 
shall  be  realized  in  the  development  of  the  indi- 
vidual, is  determined  by  external  conditions."  l 

This  must  not  to  be  taken  to  mean  that  it  is  not 
possible  to  produce  hereditable  modifications  in 
plants,  the  contrary  is  true,  but  merely  that  very 
striking  changes  may  be  of  no  lasting  importance 
because  they  are  not  transmissible. 

Of  late  years,  a  host  of  experiments  have  been 
performed  upon  animals,  the  larger  number  of  them 
with  the  object  of  determining  whether  new  char- 
acters, acquired  during  the  lifetime  of  the  parents, 
can,  under  any  circumstances,  be  transmitted  to  the 
offspring.  This  is  one  of  the  most  hotly  disputed 
questions  of  modern  biology  and  our  whole  concep- 
tion of  the  efficient  factors  which  have  brought  about 
evolution  hinges  upon  the  answer  to  this  question. 

1  Karl  Giesenhagen,  in  Die  Abstammungslehre,  pp.  307-8. 


152  THE  THEORY  OF  EVOLUTION 

The  same  experiments  are  interpreted  in  diamet- 
rically opposite  senses  by  different  writers  according 
to  their  predisposition  and  general  point  of  view. 
At  the  present  time,  it  is  probable  that  a  very  con- 
siderable majority  of  zoologists  and  botanists,  espe- 
cially in  this  country,  are  inclined  to  deny  the 
hereditary  transmission  of  characters  acquired  in  the 
post-embryonic  life  of  the  parents,  but  the  problem  is 
still  far  from  definite  solution.  Important  as  this 
problem  is  in  any  attempt  to  explain  evolution  and 
the  manner  in  which  it  has  been  effected,  it  has  no 
bearing  upon  the  question  which  I  have  been  en- 
deavouring to  answer  in  these  lectures,  as  to  the 
probable  truth  of  the  evolutionary  theory.  That 
theory  is  held  quite  as  strongly  by  those  who  deny 
as  by  those  who  affirm  the  transmission  of  acquired 
characters.  Whatever  interpretation  be  put  upon 
the  significance  of  the  experiments,  presently  to  be 
mentioned,  as  to  the  problem  of  acquired  characters, 
they  do,  at  all  events,  show  that  hereditarily  trans- 
missible modifications  may  be  artificially  produced 
in  both  animals  and  plants. 

We  are  again,  as  in  every  one  of  the  preceding 
lectures,  confronted  by  the  difficulty  of  making  an 
instructive  and  convincing  selection  from  a  great  and 
ever  growing  mass  of  facts,  but  it  is  impossible  to 
present  more  than  a  few  typical  examples.  Among 
the  older  experiments  may  be  mentioned  those  made 
by  Schmankewitsch  upon  the  brine-shrimp.  By 
increasing  the  proportion  of  salt  dissolved  in  the 


EVIDENCE  FROM  EXPERIMENT          153 

water,  the  little  creatures  passed,  in  the  course  of 
several  generations,  into  quite  a  different  form, 
identical  with  what  had  previously  been  regarded  as  a 
distinct  species.  On  the  other  hand,  by  diminishing 
the  quantity  of  salt,  and  keeping  many  successive 
generations  in  progressively  freshened  water,  the 
brine-shrimp  gradually  assumed  the  characters  of 
quite  a  distinct  genus,  which  lives  in  fresh  water.  A 
very  important  distinction  between  these  results  and 
those  obtained  by  Bouvier  with  plants,  is  that  in  the 
latter  case  the  change  was  effected  immediately  and 
went  no  farther  in  succeeding  generations,  but  could 
be  at  once  reversed  by  restoring  the  plant  to  its 
original  place  of  growth.  With  the  brine-shrimps,  on 
the  contrary,  the  changes  were  gradual  and  cumula- 
tive, many  generations  being  required  to  bring  them 
to  full  development. 

By  keeping  water-fleas  in  the  dark,  Kapterew 
caused  them  to  grow  blind,  the  eyes  losing  their 
regular  outline,  and  their  black  pigment  began  to 
wander  over  the  head  and  body.  At  first  this  dete- 
rioration appeared  sporadically,  but  after  a  year  and 
a  quarter  all  the  individuals  were  similarly  affected 
and  the  blindness  appeared  to  be  hereditary. 

Many  experiments  have  been  made  by  Weismann, 
Standfuss,  Fischer,  Schroder,  Pictet  and  others  upon 
the  caterpillars  and  cocoons  of  moths  and  butterflies, 
changing  their  food-plants,  in  some  cases,  and  in 
others  hatching  the  full-grown  insects  from  cocoons 
which  had  been  exposed  to  abnormal  conditions  of 


154  THE  THEORY  OF  EVOLUTION 

temperature.  Some  very  striking  changes  were 
produced  in  this  manner  and  the  remarkable  case  of 
the  Lunar  Moth,  taken  from  Texas  to  Switzerland, 
has  already  been  described  in  another  connection. 
(See  p.  39.)  Cocoons  of  one  species  of  butterfly 
and  one  moth,  when  exposed  to  a  low  temperature, 
considerably  below  the  freezing  point,  yielded  mature 
insects  which,  especially  in  the  males,  were  darker  in 
colour  and  had  a  different  wing-pattern  from  those 
which  were  hatched  at  normal  temperatures.  The 
second  generation,  derived  from  these  modified 
parents,  but  hatched  from  cocoons  exposed  to  the 
normal  temperature  of  the  season,  had  a  consider- 
able number  of  insects  which  resembled  their  dark 
parents,  while  others  had  reverted  to  the  ordinary 
condition  of  the  species.  In  another  moth  a  very 
decided  darkening  of  the  wings  and  a  change  of 
pattern  was  produced  by  the  opposite  method  of 
keeping  the  cocoons  in  an  atmosphere  much  hotter 
than  that  of  the  hatching  season  out  of  doors.  Here 
also  a  considerable  number  of  moths  in  the  second 
generation  inherited  the  characters  of  their  modified 
parents,  though  they  themselves  had  been  hatched 
at  the  normal  temperature.  The  larvae  of  a  beetle, 
which  feed  upon  a  smooth-leaved  willow,  were 
transferred  to  another  species  with  densely  hairy 
leaves.  The  descendants  freely  chose  the  hairy 
leaves,  when  both  kinds  were  accessible  and  the 
number  so  choosing  increased  with  each  successive 
generation,  the  effect  being  cumulative. 


EVIDENCE  FROM  EXPERIMENT          155 

A  very  remarkable  series  of  experiments  upon  the 
Colorado  Potato-beetle  were  made  by  Professor 
Tower,  of  Chicago,  under  conditions  of  the  greatest 
exactitude  and  rigorous  control,  as  much  so  as  in 
any  chemical  or  physical  laboratory;  the  results  are 
famous  throughout  the  biological  world.  In  these 
experiments  the  larvae  and  the  beetles  were  subjected 
to  varying  conditions  of  heat  and  moisture  and  at 
different  stages  of  their  development,  and  thus 
several  distinct  races  were  established,  differing  in 
size,  colour,  markings  and,  in  one  case,  in  breeding 
habits;  the  new  characters  are  hereditary  and  trans- 
mitted from  generation  to  generation.  A  remarkable 
feature  of  these  experiments  was  the  discovery  that 
the  beetles  had  a  sensitive  period  in  their  develop- 
ment, when  they  are  particularly  susceptible  to  the  in- 
fluence of  changes  in  the  environment.  When  larvae 
in  their  sensitive  period  are  exposed  to  altered  condi- 
tions, the  adult  beetles  display  corresponding  changes, 
but  these  are  not  hereditary  and  do  not  reappear 
in  the  next  generation.  When,  however,  the  beetles, 
at  the  time  when  the  eggs  are  maturing  and  getting 
ready  for  fertilization,  are  so  exposed,  the  beetles 
themselves  show  no  visible  change,  but  the  offspring 
do  and  these  modifications  are  hereditary.  Tower  in- 
terprets this  as  being  the  action  of  external  condi- 
tions directly  upon  the  reproductive  cells,  or  "germ- 
plasm,"  but  this  interpretation  is  disputed  by  others. 

Kammerer  has  made  numbers  of  experiments  on 
amphibians  and  has  succeeded  in  producing  hered- 


156  THE  THEORY  OF  EVOLUTION 

itary  modifications  in  several  of  them,  of  which  a 
few  may  be  mentioned  here.  A  salamander,  black 
with  yellow  spots,  which  lives  in  damp  woods,  is 
viviparous  and  produces  sixty  to  seventy  gill-bearing 
larvae,  which  are  deposited  in  water  and  develop 
there  for  several  months,  leaving  the  water  for  the 
land  at  the  time  of  metamorphosis,  when  the  larvae 
become  adult,  air-breathing  salamanders.  If  captive 
animals  are  deprived  of  water  in  which  to  deposit 
their  young,  a  very  few  are  produced,  but  these  are 
not  larvae,  but  little  salamanders  like  their  parents. 
When  this  second  generation  breeds,  if  it  is  allowed 
access  to  water,  it  will  once  more  produce  larvae,  but 
very  large  ones,  which  leave  the  water  in  a  few  days, 
instead  of  remaining  in  it  for  several  months.  If 
adult  salamanders  of  this  species  are  kept  for  years 
on  yellow  soil,  the  yellow  spots  and  blotches  greatly 
enlarge,  encroaching  on  the  black  ground,  and  the 
effects  of  this  treatment  are  much  increased  in  the 
second  generation,  showing  a  cumulative  action. 
The  larvae  of  a  toad  were,  by  means  of  darkness, 
cold  water,  and  other  unnatural  conditions,  made  to 
delay  their  metamorphosis  and  became  sexually  ma- 
ture and  able  to  breed  while  in  the  tadpole  stage. 
These  tadpoles  attained  great  size  and  their  offspring 
did  not  advance  beyond  the  stage  in  which  the  hind- 
legs  are  developed,  remaining  in  that  condition  for 
years  and  showing  no  tendency  to  metamorphosis. 
Other  experimenters  have  failed  to  repeat  these  re- 
sults, which  are  strongly  questioned. 


EVIDENCE  FROM  EXPERIMENT          157 

All  of  these  experiments,  and  many  more  that 
might  be  cited,  would  seem  to  prove  that  it  is  pos- 
sible to  effect  lasting  and  hereditary  modifications  in 
animals  by  abrupt  changes  in  their  environment,  and 
the  same  result  has  been  obtained  with  plants.  By 
injecting  various  substances  into  the  young  seed- 
capsule,  when  the  germs  are  becoming  ready  for  fer- 
tilization by  the  pollen,  MacDougal  effected  some 
very  remarkable  changes,  which  were  hereditary 
through  four  or  more  generations  and  remained  sta- 
ble, the  plants  exhibiting  no  tendency  to  return  to 
the  parent  type.  Gager  also  produced  hereditary 
modifications  in  plants  by  exposing  the  seed-capsule 
before  fertilization  to  the  action  of  radium,  but  these 
results  are  not  yet  conclusive. 

Thus,  the  experimental  proof  goes  to  show  that  the 
species  of  animals  and  plants  are  not  fixed  and 
immutable  entities,  but  are  capable  of  extensive 
modification  even  in  the  short  time  which  is  at  the 
command  of  the  experimenter. 

In  the  first  lecture  it  was  stated  that  the  experi- 
ments of  the  Austrian  monk,  Johann  Gregor  Mendel, 
published  in  1866,  had  attracted  no  attention,  because 
they  were  too  far  ahead  of  their  time  and  their  far- 
reaching  significance  was  not  perceived.  Redis- 
covered in  1900,  Mendel's  results  have  been  enor- 
mously extended  and  have  led  to  the  development  of 
the  new  science  of  Genetics.  This  is  an  extremely 
complex  subject,  which  would  require  more  than  one 
course  of  lectures  for  its  elucidation,  not  merely  the 


158  THE  THEORY  OF  EVOLUTION 

fraction  of  a  single  lecture,  in  which  only  a  hasty  and 
most  inadequate  sketch  can  be  attempted.  On  the 
other  hand,  it  is  not  possible  to  omit  all  mention  of 
Mendel's  work  from  a  review  of  the  modern  state  of 
opinion  on  the  subject  of  evolution,  for  this  work  has 
thrown  great  light  upon  the  mechanism  of  heredity 
and,  as  has  been  repeatedly  insisted,  heredity  is  an 
all-important  part  of  evolution. 

Mendel  crossed  two  varieties  of  peas,  one  of  which 
constantly  had  white  flowers  and  the  other  violet 
flowers.  The  first  generation  of  hybrid  plants  all 
had  violet  blossoms,  which  Mendel  explained  by 
saying  that  in  this  particular  cross  the  violet  colour 
was  "dominant"  over  the  white,  which  was  "re- 
cessive." The  second  generation  of  the  hybrids, 
however,  showed  a  change,  only  three-fourths  of  the 
plants  had  violet  blossoms  and  one-fourth  had  white; 
the  latter,  in  all  subsequent  generations,  gave  only 
white  flowers;  the  plants  had  "reverted"  to  the  origi- 
nal white  parent.  Of  the  violet-flowered  plants  in  the 
next  and  all  following  generations,  one-third  re- 
mained constantly  violet  and  two-thirds  split  in  the 
next  generation  into  three-fourths  violet  and  one- 
fourth  white.  From  the  mere  inspection  of  a  violet- 
flowered  plant,  it  was  impossible  to  say  whether  it 
would  give  rise  only  to  violet  offspring,  or  to  those 
which  were  partly  violet  and  partly  white.  These 
and  thousands  of  other  experiments  indicate  that  an 
organism  is  a  mosaic  of  "unit  characters,"  which  are 
transmitted  independently  of  one  another  to  the 


EVIDENCE  FROM  EXPERIMENT          159 

offspring,  and  that  in  each  generation  of  hybrids  a 
process  of  segregation  goes  on,  by  which  a  definite 
proportion  of  plants  is  separated  out  from  the  mass  of 
cross-bred  individuals  and  returns  to  the  character  of 
one  or  other  original  parent.  When  this  segregation 
has  once  been  effected,  all  the  subsequent  progeny  is 
constant  to  type;  the  descendants  of  the  segregated 
white  flowers  are  invariably  white. 

The  process  is  made  simpler  and  more  intelligible, 
when  the  somewhat  confusing  effects  of  dominance 
are  absent,  as  they  are  in  the  Jalap,  or  Wonder  of 
Peru,  experimented  on  by  Correns.  If  the  white- 
flowered  race  of  this  plant  is  crossed  with  the  red- 
flowered  race,  all  the  plants  of  the  first  hybrid  gener- 
ation have  uniformly  pink  flowers,  because  neither 
color  is  dominant  over  the  other  and  the  two  seem 
to  blend,  though  they  do  not  actually  do  so.  When 
the  second  generation  is  produced,  either  by  self- 
fertilization  or  by  crossing  the  hybrids  with  each 
other,  one-fourth  of  this  second  generation  is  red, 
one-fourth  white  and  one-half  pink,  and  each  suc- 
ceeding generation  acts  in  the  same  way. 

This  behaviour  is  explained  by  assuming  that 
every  unit  character  is  represented  in  the  reproduc- 
tive cells,  or  germ,  by  something  which  causes  the 
development  of  that  character  in  the  full-grown 
plant  and  this  something  is  called  the  "factor"  for 
the  character.  If  we  symbolize  the  factor  for  the 
red  colour  by  R  and  that  for  the  white  by  W,  then 
the  germs  of  the  red-flowering  race  contain  the  fac- 


160  THE  THEORY  OF  EVOLUTION 

tor  R  and  those  of  the  white-flowering  race  the  factor 
W.  The  female  hybrids  produce  egg-cells,  one-half  of 
which  contain  R  and  one-half  W;  likewise  the  male 
hybrids  produce  pollen-cells,  one-half  of  which  contain 
R  and  one-half  W.  Fertilization  consists  in  the  fusion 
of  the  nucleus  of  the  male  cell  with  that  of  the  female 
cell,  as  has  been  very  often  directly  observed  in  all 
kinds  of  animals  and  plants  which  have  the  sexual 
method  of  reproduction.  After  fertilization  the  germ, 
which  is  now  ready  to  begin  its  development,  contains 
factors  derived  from  both  parents  and,  according  to 
the  mathematical  laws  of  probability  and  combina- 
tion, there  would  be  four  combinations:  RR,  WR, 
RW,  and  WW.  The  factor  from  the  male  parent  is  put 
first,  so  that  the  combination  RW  means  that  the 
red  factor  is  derived  from  the  male  and  the  white 
factor  from  the  female;  in  WR  this  is  reversed, 
though,  in  practice,  the  result  is  usually  the  same  in 
either  case.  In  the  Jalap,  where  neither  white  nor 
red  is  dominant,  both  RW  and  WR  give  the  pink 
flowers  and,  obviously  in  the  proportion  of  one-half 
the  entire  number  of  plants  in  the  second  hybrid  gen- 
eration. The  germs  having  the  factors  RR  give  the 
segregated  red  flowers  and,  as  there  is  no  W,  it  is 
plain  that  all  the  descendants  of  the  RR  seeds, 
generation  after  generation,  if  crossing  is  avoided, 
will  produce  only  red  flowers.  For  the  same  reason, 
the  WW  germs  and  all  their  pure-bred  descendants 
can  give  only  white  flowers.  If  the  unit-characters 
do  not  actually  blend  and  become  inseparable,  it  is 


EVIDENCE  FROM  EXPERIMENT         161 

easy  to  see  why  segregation  must  occur  in  each  gener- 
ation of  the  hybrids. 

The  complexity  of  the  calculation  rises  very  rap- 
idly, when,  instead  of  experimenting  with  only  a 
single  pair  of  alternative  characters,  as  in  the  cases 
previously  cited,  combinations  of  several  such  pairs 
are  effected.  Mendel  himself  used  as  many  as  seven 
pairs,  combined  in  many  different  ways.  Yet,  if 
regard  be  paid  only  to  the  behaviour  of  a  single  pair 
of  alternatives,  it  makes  no  difference  how  many 
other  unit-characters  are  associated  in  the  combina- 
tion, as  they  all  act  independently.  When  the  be- 
haviour of  all  the  characters  is  analyzed,  it  is  found 
that  the  mathematical  laws  of  probability  and  com- 
bination are  followed  as  closely  as  when  but  a  single 
pair  is  dealt  with.  Indeed,  the  correspondence  be- 
tween the  calculated  and  the  experimental  results 
is  highly  remarkable. 

Mendel's  own  experiments  were  made  with  plants, 
but  the  same  laws  of  heredity  apply  to  animals. 
Lang  experimented  with  the  garden-snail  of  Europe, 
one  race  of  which  has  a  yellow,  unstriped  shell  and 
another  has  the  shell  with  spiral  stripes  of  black. 
Crossing  these,  the  unstriped  yellow  proved  to  be 
dominant,  all  the  individuals  of  the  first  hybrid 
generation  having  shells  of  that  kind.  In  the  second 
generation  segregation  began  and  in  just  the  same 
proportion  as  in  the  violet  and  white  peas;  one-fourth 
of  the  shells  were  striped,  three-fourths  unstriped. 
All  the  descendants  of  the  former  were  constantly 


162  THE  THEORY  OF  EVOLUTION 

striped,  but  the  yellow  forms  again  split  in  the  next 
generation.  Inasmuch  as  the  sexual  method  of  re- 
production was  independently  acquired  in  both  the 
vegetable  and  the  animal  kingdoms,  not  inherited 
from  an  ancestor  common  to  both,  this  agree- 
ment indicates  that  the  Mendelian  laws  of  inheri- 
tance are  of  universal  validity,  save  under  such  cir- 
cumstances as  will  be  mentioned  later. 

The  Mendelian  laws  further  explain  how  new 
races  may  arise  through  the  sudden  loss  of  one  or 
more  factors  by  mutation.  The  common  house- 
mouse  in  its  wild  state  has  a  characteristic  gray 
colour,  but  there  are  several  fancy  breeds  of  mice, 
white,  brown,  black  and  blotched,  all  of  which  were 
derived  from  the  wild  form.  Many  experiments  in 
the  cross-breeding  of  these  domesticated  varieties 
with  one  another  and  with  their  wild  ancestor,  in  all 
sorts  of  combinations,  have  shown  that  the  varieties 
have  arisen  through  the  suppression  of  factors. 
Many  factors  are  involved  in  these  cross-breedings, 
but,  simplifying  the  matter  to  the  utmost,  they  may 
be  reduced  to  four.  (1)  A  colour-factor,  which  must 
be  present  if  any  colour  at  all  is  to  be  developed  in 
the  hairs  of  the  coat.  (2)  A  factor  which  determines 
that  the  colour  shall  be  distributed  over  the  whole 
body.  (3)  A  pigment-factor,  which  conditions  the 
occurrence  of  black  in  the  hairs,  which,  without  it, 
would  be  brown.  (4)  A  distribution-factor,  which 
controls  the  arrangement  of  black  and  brown  in  each 
hair,  so  that  the  combined  effect  is  the  characteristic 


EVIDENCE  FROM  EXPERIMENT          163 

mouse-colour.  If  this  last  factor  should  be  elim- 
inated, the  result  would  be  the  production  of  black 
mice,  an  explanation  which  is  strongly  supported  by 
crossing  black  mice  with  the  normal  animal  and 
making  a  mathematical  analysis  of  the  proportion- 
ate numbers  of  hybrid  forms.  If  the  pigment- 
factor,  which  determines  the  formation  of  the  black 
colour,  should  disappear  in  black  mice,  brown  ani- 
mals would  be  produced  and,  if  the  factor  control- 
ling the  distribution  of  colour  should  be  lost,  then 
blotched  mice  would  appear.  Finally,  the  suppres- 
sion of  the  general  colour-factor  would  lead  to  the 
development  of  white  or  albino  mice.  We  may 
thus  witness  the  formation  of  new  varieties  through 
the  loss  of  factors,  but  the  addition  of  new  factors 
has  not  yet  been  observed,  unless  the  additional  toe 
in  Castle's  four-toed  race  of  guinea  pigs  should  be 
so  interpreted. 

Interesting  and  profoundly  important  as  are  the 
results  of  the  Mendelian  investigations,  it  must  be 
admitted  that,  so  far,  they  have  rendered  but  little 
assistance  in  making  the  evolutionary  processes 
more  intelligible  and,  instead  of  removing  difficul- 
ties, they  have  rather  increased  them.  That  the 
Mendelian  laws  of  inheritance  are  immensely  valu- 
able to  breeders,  is  not  to  be  questioned,  but  it  re- 
mains to  be  determined  just  how  far  they  are  oper- 
ative in  nature  and  what  bearing  they  have  upon 
the  problems  of  evolution.  The  experimenter  can, 
if  he  so  desires,  preserve  all  of  his  plants  or  animals 


164  THE  THEORY  OF  EVOLUTION 

through  many  successive  generations,  deriving 
thousands  of  individuals  from  a  single  pair  of  pro- 
genitors, but,  under  natural  conditions,  matters  are 
very  different.  A  single  plant  may  produce  thou- 
sands of  seeds  in  a  season,  though  but  a  very  few,  or, 
perhaps  only  one,  can  arrive  at  maturity  and  repro- 
duce in  its  turn.  Unless  it  possesses  some  very 
notable  advantage,  the  determination  of  which  in- 
dividual seedling  shall  survive,  is  far  more  likely  to 
be  conditioned  by  the  spot  where  it  happened  to 
germinate,  with  reference  to  light,  moisture,  etc., 
than  by  its  germinal  constitution.  It  is  difficult  to 
see  how  the  laws  of  proportionate  segregation  can 
have  any  effect  under  such  conditions.  Some  stu- 
dents of  genetics  go  so  far  as  to  maintain  that  all 
evolutionary  changes  are  made  by  the  elimination 
of  factors,  and  others  that  species  are  immutable,  ex- 
cept as  new  combinations  are  formed  through  hybrid- 
ization. 

It  is  quite  possible  that  the  Mendelian  laws  are 
those  of  ordinary,  conservative  inheritance,  by 
means  of  which  species  are  maintained  in  their 
normal  condition,  and  yet  may  not  be  applicable, 
when  new  characters  arise  through  the  action  of  the 
environment.  This  is  suggested  by  Tower's  experi- 
ments on  potato-beetles,  cited  above.  He  says: 
"In  what  way  is  the  constitution  of  the  germ  cell 
modified  so  that  the  organism  shows  in  subsequent 
generations  a  permanent  change  in  its  coloration? 
It  has  been  pointed  out  to  me  by  Professor  Morgan 


EVIDENCE  FROM  EXPERIMENT         165 

that  in  those  experiments  the  behavior  of  the  first 
generation  is  difficult  of  interpretation.  In  these 
experiments  the  male  cells  and  the  female  cells,  and 
sometimes  both,  have  been  subjected  to  conditions 
of  experiment  at  a  susceptible  stage. 

"Morgan  has  raised  the  question,  why  do  indi- 
viduals, developed  from  eggs  which  have  been  sub- 
jected to  conditions  of  experiment  and  fertilized 
with  normal  sperm,  not  give  a  subsequent  hybrid 
behavior?  No  hybrid  splitting  has  ever  been  found 
in  any  of  my  experiments,  or  in  those  of  MacDougal 
or  Gager.  The  resulting  modification  reproduces 
itself  true  to  type,  and  does  not  give  subsequent 
splittings  suggestive  of  the  combination  of  different 
factors  or  unit-characters.  If  there  are  unit-char- 
acters, it  is  logical  to  expect  that  in  experiments  of 
this  kind  the  experiment  would  modify  the  unit- 
character  in  the  germ-plasm,  and  that  this  modified 
unit-character  would  then  behave,  when  crossed 
with  its  normal  homologue,  exactly  as  hybrids  do  in 
other  cultures.  The  total  lack  of  this  behavior  in 
my  experiments,  and  those  of  MacDougal,  Gager, 
and  others,  might  be  considered  good  evidence  that 
there  are  no  such  things  as  unit-characters,  nor  in 
the  germ  cells  any  potentiality  capable  of  individual 
removal  or  behavior.  Any  such  deduction,  however, 
is  unwarranted  and  contrary  to  known  facts,  and, 
furthermore,  these  modified  characters  themselves 
show  that  after  establishment  [italics  mine]  they  are 
alternative  and  capable  in  many  instances  of  re- 


166  THE  THEORY  OF  EVOLUTION 

placement  and  recombination  in  full  conformity 
with  established  principles  of  heredity  behavior."  l 

These  and  many  similar  questions  must  be  left 
to  future  investigations  for  reply;  it  is,  in  any  event, 
a  great  advance  that  we  should  be  able  to  approach 
these  problems  by  means  of  exact  experiment  and 
mathematical  analysis. 

This  concludes  our  sketch  of  the  various  lines  of 
testimony  which  have  been  adduced  in  favour  of 
the  theory  of  evolution  and  I  can  readily  imagine 
that  some  of  you,  at  least,  are  saying  to  yourselves: 
"Is  that  all?  I  don't  think  that  amounts  to  much!" 
For  such  lack  of  conviction,  if  such  there  be,  there 
are  obvious  reasons.  It  has  been  necessary,  because 
of  the  very  brief  time  at  our  disposal,  only  six  hours 
all  told,  to  make  a  very  limited  selection  out  of  the 
great  mass  of  evidence  that  might  otherwise  have 
been  brought  forward.  However  candidly  and  skil- 
fully the  selection  may  be  made,  it  cannot  produce 
the  same  impression,  as  when  the  whole  body  of 
testimony  can  be  presented,  a  principle  which  is 
well  understood  in  the  courts  of  justice.  I  have 
said  very  little  with  regard  to  plants,  picking  out 
only  a  few  conspicuous  examples,  yet  the  evidence 
in  their  case  is  the  same  as  for  animals,  fuller  and 
clearer  in  some  respects,  less  so  in  others.  Professor 
Giesenhagen  sums  up  for  plant  evolution  thus:  "The 
evidence  disclosed  by  palaeontology  shows  us  that 

1  Wm.  Laurence  Tower,  in  Heredity  and  Eugenics,  Chicago,  1912, 
pp.  £2£~~3. 


EVIDENCE  FROM  EXPERIMENT          167 

the  species  of  plants  which  inhabit  the  earth  to-day 
have  proceeded  from  the  different  plant-forms  of 
the  earth's  earlier  epochs.  Experimental  investiga- 
tion and  observation  upon  heredity  in  existing  or- 
ganisms prove  that  the  specific  characters  of  plants 
are  changeable,  that  especially  by  hybridization  and 
mutation  new  species  of  plants  may  arise  from  those 
already  present.  The  comparative  study  of  struc- 
ture and  course  of  development  in  plants  justifies 
us  in  concluding  that  the  species  of  plants,  under 
the  continually  acting  influence  of  the  external 
conditions  of  life,  lose  certain  possibilities  of  develop- 
ment and  gain  new  possibilities  and  may  thereby 
undergo  a  genealogical  development."  1 

I  must  again  remind  you  that  what  we  are  en- 
deavouring to  prove  is  a  great  historical  process, 
which  has  gone  on  through  vast  and  unnumbered 
aeons  of  time,  when  no  observer  was  present  to 
record  the  forward  steps  in  the  magnificent  proces- 
sion of  life.  The  details  of  evolutionary  change 
must  have  been  of  an  unimaginable  complexity  and, 
of  necessity,  the  evidence  for  that  change  must  be 
chiefly  indirect  and  circumstantial,  to  appreciate 
the  full  force  of  which  requires  a  certain  degree  of 
technical  training.  The  Indian  hunter  swiftly  fol- 
lows a  trail  which  is  invisible  to  the  white  man's 
eye,  and  the  experienced  detective  finds  clues  where 
the  average  man  sees  nothing  of  importance,  so  only 
one  who  has  gained  some  first-hand  knowledge  of 

1  Karl  Giesenhagen,  in  Die  AbstaHKOungslehre,  p  320. 


168  THE  THEORY  OF  EVOLUTION 

living  things  is  in  a  position  to  estimate  the  strength 
and  the  weakness  of  such  testimony  as  has  passed 
before  us.  That  does  not  for  a  moment  imply  that 
we  are  here  dealing  with  esoteric  mysteries  which  the 
laity  are  unable  to  comprehend  and  must  accept  on 
faith.  The  detective's  work  must  convince  the 
jury  and  in  this  case,  the  jury  is  the  body  of  intelli- 
gent, non-professional  opinion.  I  merely  wish  to 
point  out  that  such  an  exposition  as  I  have  been 
able  to  make,  in  elementary  and  non-technical 
fashion,  is  less  convincing  than  the  whole  body  of 
known  facts  is  to  those  who  are  familiar  with  it. 

From  the  very  nature  of  the  case,  complete  de- 
monstration is  impossible;  we  can  only  determine 
which  one  of  alternative  explanations  is  most  in 
harmony  with  observation  and  best  explains  all  the 
facts,  which  one  is  therefore  the  most  probable. 
What  gives  great  weight  to  the  evidence  in  support 
of  the  evolutionary  theory  is  the  harmonious  con- 
currence of  so  many  independent  lines  of  testimony. 
Whether  we  deal  with  classification,  or  the  results 
of  domestication,  with  comparative  anatomy,  em- 
bryology, blood  tests,  palaeontology,  geographical 
distribution,  or  experimental  investigation,  we  find 
in  every  instance  that  the  simplest,  most  satisfactory 
and  least  forced  interpretation  is  that  which  is 
offered  by  the  theory  of  evolution.  The  probability 
rises  in  geometrical  ratio  with  each  additional,  in- 
dependent class  of  evidence. 

The  pathway  of  science  is  strewn  with  the  wrecks 


EVIDENCE  FROM  EXPERIMENT          169 

of  hypotheses  and  theories,  which  have  served  their 
purpose  and  have  been  worn  out  and  discarded. 
How  are  we  to  distinguish  between  those  theories 
which  will  be  permanent  and  those  which  are 
doomed  to  decay  and  oblivion?  A  theory  is  only 
an  attempt  to  explain  and  interpret  a  body  of  facts 
and,  for  a  time,  this  may  be  equally  well  done  by  a 
true  theory  and  a  false  one;  both  may  seem  equally 
plausible  and  promising.  It  is  the  advance  of  dis- 
covery, bringing  to  light  many  new  facts,  which 
were  neither  known  nor  anticipated  when  the  rival 
theories  were  first  propounded,  that  puts  the  theories 
to  the  severest  test.  From  this  point  of  view,  the 
doctrine  of  evolution  has  stood  the  test  remarkably 
well.  Darwin's  book  put  new  life  into  all  depart- 
ments of  biology;  every  one,  friends  and  foes  alike, 
was  eager  to  find  proofs  which  should  establish  or 
overthrow  this  new  and  innovating  belief,  so  that 
all  lines  of  inquiry  were  pushed  forward  with  re- 
newed activity  and  enthusiasm.  This  zealous  labour 
has  resulted  in  the  accumulation  of  a  mass  of  data 
such  as  Darwin  never  dreamed  of.  The  existence 
of  the  marvellous  treasures  which  have  been  disin- 
terred from  the  rocks  of  western  North  America 
was  hardly  suspected  when  the  "  Origin  of  Species  " 
appeared,  yet  it  is  there  that  were  found  some  of  the 
most  complete  genealogical  series  yet  made  known. 
Though  Darwin  himself  was  one  of  the  pioneers  in 
the  discovery  of  South  American  fossil  mammals, 
it  was  not  until  after  his  death  that  the  immense 


170  THE  THEORY  OF  EVOLUTION 

wealth  of  the  southern  continent  was  brought  to 
light.  The  long  isolation  of  South  America  made  it 
the  arena  of  a  vast  experiment  in  evolution,  which 
may  be  followed  in  the  most  gratifying  manner  and 
which  has  solved  some  of  the  most  puzzling  prob- 
lems of  modern  distribution.  South  Africa,  Russia, 
China  and  the  Malay  Archipelago  have  also  yielded 
to  the  explorer  much  of  which  Darwin  knew  nothing, 
but  which  he  would  have  cordially  welcomed  as 
strongly  supporting  his  belief.  It  is  the  same  in 
other  fields  of  research,  new  and  unexpected  dis- 
coveries are  continually  being  made,  but  evolution 
still  affords  the  best  explanation  of  them.  A  re- 
markable instance  of  this  is  the  blood  tests,  of  which 
an  account  was  given  in  the  third  lecture.  Here 
was  a  totally  new  field,  the  existence  of  which  was 
altogether  unsuspected,  yet,  when  the  field  was 
opened,  it  afforded  some  of  the  most  cogent  evidence 
in  favour  of  the  evolutionary  conception  that  has 
anywhere  been  found.  Not  that  new  difficulties  and 
perplexities  have  not  arisen,  but  in  spite  of  these, 
the  probability  of  the  theory  remains  unshaken  after 
more  than  hah*  a  century  of  unceasingly  active 
investigation  carried  on  all  over  the  world.  The 
doctrine  seems  stronger  now  and  is  upheld  by  a 
greater  proportion  of  naturalists  than  when  it  made 
its  early  conquests  of  opinion. 

Another  searching  test  of  a  theory  is  when  it  can 
be  made  the  basis  of  prediction  or  deduction.  That 
an  astronomer  should  be  able  accurately  to  predict 


EVIDENCE  FROM  EXPERIMENT          171 

an  eclipse  is  strong  proof  that  the  theory  of  lunar 
and  terrestrial  motions,  from  which  he  makes  his 
calculations,  is  sound.  From  irregularities  in  the 
motion  of  Uranus,  the  outermost  planet  then  visible, 
the  astronomers  predicted  the  discovery  of  another 
planet  beyond,  and  the  prediction  was  verified  when 
Neptune  was  discovered  with  the  telescope.  This 
was  rightly  accepted  as  convincing  proof  that 
the  theory  of  the  solar  system  and  of  planetary 
motions,  from  which  these  distinguished  men  had 
worked,  was  true.  Similarly,  predictions  and  deduc- 
tions have  been  made  and  subsequently  verified  con- 
cerning "missing  links"  in  genealogical  series.  I  my- 
self had  the  great  pleasure  of  finding  in  the  rocks  of 
northern  Utah  a  fossil  animal  with  a  type  of  denti- 
tion which  no  man  had  ever  seen,  but  which  had  been 
predicted  some  time  before  by  Dr.  Max  Schlosser 
of  Munich.  That  such  predictions  can  be  and  have 
been  successfully  made,  is  very  strong  evidence  that 
the  theory  of  evolution,  which  is  assumed  as  the 
basis  of  deduction,  is  well  founded. 

Admitting  fully  all  that  has  been  said,  we  must 
yet  beware  of  erecting  evolution  into  a  sacred  dogma 
which  no  one  shall  dare  to  criticise  or  doubt.  Huxley 
has  rightly  said  that  "Science  commits  suicide  when 
she  adopts  a  creed,'*  and  it  is  a  duty  to  keep  the 
mind  ever  open  for  new  and  revolutionary  discover- 
ies, which  may  change  the  aspect  and  significance  of 
all  previously  acquired  knowledge.  While  I  believe 
that  the  evolutionary  conception  of  nature  is  one 


172  THE  THEORY  OF  EVOLUTION 

of  our  permanent  possessions  and  that  it  will  in  the 
future  continue  to  direct  and  condition  all  lines  of 
intellectual  inquiry  and  advance,  I  can  understand 
that  half  a  century  hence  the  question  may  possibly 
have  assumed  a  very  different  aspect.  Once  more 
quoting  from  Professor  Giesenhagen:  "As  soon  as 
direct  experience  brings  to  light  unambiguous  nega- 
tive examples,  which  can  in  no  way  be  harmonized 
with  our  conclusions  from  analogy  and  induction, 
we  must  be  ready  to  make  a  corresponding  change 
in  our  doctrines  and  bring  them  into  harmony  with 
the  newly  recognized  truth,  for  infallibility  in  mat- 
ters of  science  is  given  to  no  man.  Up  till  now, 
however,  no  such  negative  examples  are  known."  l 

1  K.  Giesenhagen,  loc.  cit. 


INDEX 


N.  B.  Entries  in  quotation  marks  are  titles  of  books.  Technical 
names  of  genera  and  species  are  in  italics;  names  of  authors  in  small 
capitals. 


Acquired  characters,  20,  151 

Adriatic  Sea,  138 

,Egean  Sea,  139 

Africa,  94,  98,  110,  121,  123,  124, 

126,  127,  128,  134,  139,  145,  170 
AGASSIZ,  88 
Agate,  Neb.,  113 
Agave,  121 

Alaska.  106,  123,  131,  132,  140 
Alps,  130 
Americas,  137 
Ammonites,  91 
Amphibia,  33,  144,  155 
Amphioxus,  65 
Ancon  sheep,  23 
ANDREW,  97 
Antarctic  continent,  90 
Ant-eaters,  134 
Antelopes,  132,  135,  140 

blood  of,  78 

mountain,  107 
Anti-body,  74 
Anti-toxin,  74 
Antilles,  142 
Ants,  89 
Apes,  blood  of,  77 

Old  World,  134 
Archaopteryx.  96,  97 


Arctic  animals  and  plants,  130 

lands,  120 

Arctogsea,  128,  136,  137 
Argentina,  109,  130,  135 
Armadillos,  134 
Asia,  104,  106,  107,  109,  110,  123, 

124,  127,  128,  129,  131,  132 
Asphalt  pits,  123 


Australia,  106,  110,  121,  127.  128, 
134,135 

Austria,  24 
Azores,  142 

Balanoglossus,  65 
Barriers,  125 
BABTLETT,  88 
Bat,  33 

wing  of,  42,  47 
BATESON,  5 
Bats,  127,  144 
Bavaria,  96 

Beagle,  H.  M.  S.,  15,  16 
Bears,  32,  131,  140 

polar,  120 

short-faced,  135 


Beetle,  larvse  of,  154 


178 


174 


INDEX 


Beetles,  Cenozoic,  89 

embryo,  68 
Belemnites,  91 
Bering  Sea,  106,  126 

Strait,  106,  126 
Bermuda,  birds  of,  146 
Binomial  nomenclature,  32 
Biogenetic  law,  57 
Bird,  wing  of,  42,  48 
Birds,  33,  144 

absence  of  from  Palaeozoic,  91 

blood  of,  79,  80 

Cenozoic,  89 

derivation  from  reptiles,  96 

descent  of,  71 


Mesozoic,  90 
ontogeny  of,  71 

Bison,  132 

Bivalves,  90 

"Blood  Immunity  and  Blood  Re- 
lationship," 76 

Blood  tests,  73,  170 

BOLL,  39 

Boreal  zone,  132 

Borneo,  124,  142,  143 

BOUVIER,  150,  153 

Brachiopoda,  Cenozoic,  89 
ontogeny  of,  65 
Palaeozoic,  92 

Brachyceratops,  119 

BRAUER,  147 

Brine-shrimp,  152 

British  Islands,  139 

Bryozoa,  65 

Buenos  Aires,  125 

BTJFFON,  94 

Bumble  bee,  53 

Butterflies,  153 


Butterflies,  Cenozoic,  89 
larva,  59 
mouth-parts  of,  53 

Cactus,  121 
California,  Lower,  128 
Cambrian  fossils,  85 

period,  85,  87 
Camels,  114 

blood  of,  78 

distribution  of,  129 

evolution  of,  114 

true,  129 

Canadian  Rockies,  85 
Canidae,  32 
Canis  lupus,  32 
Cannon-bone,  45,  114 
Cape  Verde  Ids.,  142,  145 
Carboniferous     period,     87,     95, 

96 

Caribbean  Sea,  138 
Caribou,  132 
Carnivore,  32,  133,  135 

blood  of,  78 
Carpus,  44 
Cassowary,  50 
CASTLE,  163 
Casuarina,  143 
Catastrophism,  11,  12 
Caterpillar,  59,  153 
Cats,  32,  134,  140 
Cattle,  36,  144 

wild,  121 
Cebidae,  77 
Cenogenetic,  60 
Cenozoic  era,  87 

life,  88 
Central  America,   127,    128,    129, 

131,  136,  138 


INDEX 


175 


Century  plant,  121 
Ceratops,  119 
Cercopithecidae,  77 
Cetacea,  67 
China,  170 
Chordata,  65 
Civets,  140 
Class,  32,  125 
Classification,  31 
Cloven-hoofed,  45 
COCKAYNE,  34 

Cockroach,  mouth-parts  of,  53 
Cockroaches,  Palaeozoic.  92 
Cocoons,  153 
Coleoptera,  89 
Comparative  anatomy,  42 
Conifers,  90 
Connecticut  valley,  84 
Continental  islands,  141 
CORRENS,  24,  159 
Crabs,  53 
Crayfishes,  144 
Creative  plan,  30,  88 
Cretaceous  period,  87,  118 

vegetation,  90 
Crinoids,  89 
Crocodiles,  blood  of,  78,  79 

Cenozoic,  89 
Crustacea,  51,  52 

Cenozoic,  89 

Palaeozoic,  92 
Cryptogams,  91 
Cuba,  143 

CUVIER,  7,  10,  11,  88 
Cycadofilices,  96 
Cycads,  Cenozoic,  88 

Mesozoic,  90 

Palaeozoic,  91 


DARWIN,  1,  3,  7,  8,  11,  12,  13,  14, 

15,  16,  17,  19,  20,  21,  22,  23,  25, 

26,  29,  31,  33,  35,  37,  39,  65,  83, 

85,  86,  95,   120,   141,  144,  145, 

149,  169,  170 
Darwinians,  6,  25 
Darwinism,  5,  6 
Deer,  140 

blood  of,  78 

Sonoran,  133 

South  American,  138 

Virginia,  138 
Degeneration,  72 
Devonian  period,  87 
Dinosaurs,  118 
Dipnoi,  65 
Distribution,  discontinuous,  128 

geographical,  120 
Dogs,  36 
Dolphins,  67 
Domesticated  animals  and  plant*, 

35 

Domestication,  35,  150 
Dominant  characters,  158 
Dragon-flies,  92 

East  Indies,  141 

mammals  of,  143 
Eastern  hemisphere,  133 
Egg,  62,  64 
Egg-albumins,  78 
Egypt,  94 

Elephant,  hairy,  132 
Elephants,  80,  120,  123 

history  of,  93 

true,  140 
Elk,  132 
Embryology,  57 
English  Channel,  126 


176 


INDEX 


Eocene  epoch,  87,  104,  105,  109, 

110,  117 

Eohippus,  102,  105 
Equine  family,  106 
Equus,  102,  105,  108 
Ethiopian  region,  128,  139 
Europe,   93,    104,    109,    123,    127, 

128,  129,  130,  131,  132,  138 
Eustachian  Canal,  64 
Experiment,  149 
Extinction,  122,  140 

Pleistocene,  106,  133,  135 

Factor,  159,  162,  163, 165 
Family,  32,  125 
Fan-tail,  36 
Faunas,  island,  146 
Ferns,  96 

Cenozoic,  88 

Palaeozoic,  91 
Fertilization,  160 
Fibula,  100,  104.  114, 116 
Fin-whale,  67 
FISCHER,  153 
Fishes,  33 

Cenozoic,  89 

Cretaceous,  90,  93 

fresh-water,  144 

Mesozoic,  90 

of  Panama,  138 

Palaeozoic,  91 
Fish-like  forms,  91 
Fissipedia.  32 
FLEISCHMANN.  1,  2,  29 
Florida,  128 
Flies.  89 

Flowering  plants,  91 
Flying  foxes,  48 
Foot-prints,  fossil,  84 


Formosa,  124 
Fossils,  85,  92, 123 
Fowl,  blood  of,  79 

embryo  of,  63 
Fowls,  36 
Foxes,  32,  140 
France,  10,  111,  132,  140 
Frog,  egg  of,  63 

larva  of,  59 
Frogs,  144 

GAGER,  157,  165 

Galapagos  Ids.,  15,  120,  144,  145 

Ganoids,  Cenozoic,  89 

Mesozoic,  90 

Palaeozoic,  91 
Gastropods,  90 
Gazelle-camels,  116 
GEGENBAUR,  60 
GEMMINGER,  40 
Genetics,  24,  157 
Genus,  32,  125 
Geological  record,  imperfection  of, 

83 

Georgia,  130 
Germany,  10,  24 
GIARD,  9,  10 

GIESENHAGEN,  151,  166,  167,  172 
Gill-pouches,  embryonic,  64 
Giraffe-camels,  118 
Giraffes,  140 
Glyptodonts,  133,  135 
Gnawers,  70,  103 
Goats,  144 

blood  of,  78 
GRAHAM-SMITH,  76,  77 
Grasses,  88,  103 
Great  Britain,  123,  124,  142 

Plains,  113 


INDEX 


177 


Greek  islands,  124 
Greenland,  131 
Ground-pines,  88 
Ground-sloths,  123,  133,  135 
Guanaco,  114,  115,  117,  129 
Guiana,  138 
Guinea  pigs,  163 

RECKEL,  37,  57 

Haiti,  143 

Hapalidse,  77 

Hawaiian  Ids.,  142,  146 

Hen,  egg  of,  62 

Heredity,  19 

"History  of  Creation,"  57 

Holarctic  region,  128,  131,  132,  139 

Hoofed  animals,  103,  134,  135 

Horse,  33,  46 

blood  of,  76 

fore  leg  of,  42,  43,  44,  45,  46 

mountain,  107 
Horses,  36,  123,  135 

browsing,  107 

evolution  of,  98 

Eocene,  104 

Miocene,  101,  102 

Oligocene,  103 

Pleistocene,  99,  106 

Pliocene,  101 

Recent,  99 

South  American,  106,  107 

spread  of,  125 

wild,  121 
Horse-shoe  crab,  blood  of,  79 

ontogeny  of,  65 
Horse-tails,  95 

Cenozoic,  88 

Palaeozoic,  91 
Houses,  development  of,  30 


Humerus,  43 
BUTTON,  12 
HUXLEY,  14,  171 
Hybridization,  164,  167 
Hyenas,  140 
Hymenoptera,  89 

Immutability  of  species,  11,  29,  31, 

36,  88,  120,  137 
India,  123,  129 
Insects,  144 

Cenozoic,  89 

embryo,  68 

mouth-parts  of,  53 

Palaeozoic,  92 
Invertebrates,  Cenozoic,  89 

Mesozoic,  90 

Palaeozoic,  91 
Ireland,  142 
"Island  Life,"  141 
Islands,  animals  and  plants  of,  141 

continental,  141 

oceanic,  141,  144 
Italy,  124,  139 

Jackals,  32 
Jaguar,  140 
Jalap,  159,  160 
Jamaica,  39,  121 
Japan,  124,  128 
Java,  124,  142,  143 
Jurassic  period,  87,  96,  97 

Kamchatka,  132,  140 
KAMMERER,  155 
KAPTEREW,  153 
Kentucky,  130 
Krakatoa,  143 

KUKENTHAL,  69 


178 


INDEX 


Labrador,  130 

LAMARCK,  7,  8,  9, 10, 13, 19,  22,  31, 

33 

Lamp-shells,  65,  89 
Lancelet,  65 
LANG,  161 
Languages,  55 
Larvse,  59 
Lemurs,  blood  of,  77 

eye  of,  76 
Lepidoptera,  89 
Lepus  cuniculus,  37 

huxleyi,  37 
Lichens,  88 
Lilies,  88 

LINNAEUS,  7,  11,  29,  32,  S3 
Lions,  120,  123 
Lizard,  fore  leg  of,  42 
Lizards,  144 

blood  of,  78,  79 

Cenozoic,  89 

Llama,  114,  123,  129,  134 
Lobster,  51 
Los  Angeles,  123 
Lung-fishes,  65,  89 
Lungs,  derivation  of,  64 
Lycopods,  95 

Cenozoic,  88 

Palaeozoic,  91 
LYUEKKER,  66 
LTELL,  11,  12,  13,  16 

MAA8.54 

MAcDouoAL,  157,  165 
Macula  lutea,  76 
Madagascar,  124,  128,  142 
Madeira,  142 

birds  of,  146 
Malagasy  region,  128 


Malay  Archipelago,  128,  170 
"Malay  Archipelago,  The,"  141 
Malta,  124 
MALTHUS,  17 
Mammalia,  32,  33 
Mammals,  absent  from  Palaeozoic, 
91 

ancestry  of,  4,  97 

blood  of,  78,  80 

Cenozoic,  89 

Mesozoic,  90,  98 

Tertiary,  98,  131 
Mammoth,  Siberian,  132,  140 
Man,  arm   and   hand  of,  42,  43, 
44,46 

blood  of,  74 

body  of,  80 

embryo  of,  63 

eye  of,  76 
.Pleistocene,  138 
Marmosets,  134 

blood  of,  77 
Marsupials,  127 

Australian,  134,  135 

blood  of,  78 
Maryland,  84 
Massachusetts,  84,  121 
Mastodons,  123,  135,  139 
Mediterranean  lands,  121,  140 

Sea,  59,  139 

MENDEL,  22,  24,  157,  158,  161 
Mendelian  laws,  162,  163,  164 
Mendelism,  5 
Mesohippus,  102,  105 
Mesozoic  era,  87,  90,  98,  118 
Metacarpal,  44,  45 
Mexico,  106, 128, 132,  133, 136, 138 
Mice,  127,  134,  144 

albino,  163 


INDEX 


179 


Mice,  fancy,  162 

Migration,  93 

Miocene  epoch,  87,  93,  101,  102, 

106,  107,  109,  113.  116.  133.  135 
Missing  links,  171 
Mole,  33 

fore  leg  of,  42 
Mollusca,  Cenozoic,  89 

Mesozoic,  90 

ontogeny  of,  65 

Palaeozoic,  92 
Mongoose,  121 
Monkey,  33 
Monkeys,  blood  of,  77 

eye  of,  76 

Old  World,  76,  77,  134 

South  American,  76,  77,  133 
Moose,  132 
MORGAN,  164,  165 
Morocco,  124 
Mosquito,  53 


Monoclonius,  119 
Monotremes,  127 
Moth,  Brown-tail,  121 

Gypsy,  121 

Lunar,  39,  154 
Moths,  153 

Cenozoic,  89 

mouth-parts  of,  53 
Mouse,  162 
Musk-ox,  120,  130 
Mutation,  162,  167 

Theory,  23 
Mutations,  23 

NAGEIJ,  20 

Natural  Philosophers,  10 
Selection,  14,  18,  20,  25 


Nebraska,  113 

Neo-Darwinian  school,  22 

Neogsea,  128 

Neotropical  region,  128.  131,  133, 

134,  136 
Neptune,  171 
New  England,  140 

Guinea,  127 

Hampshire,  130 

Jersey,  84 

World,  111,  140 

Zealand,  34,  89,  142 
Newts,  144 

North  America,  93,  104,  107, 
109,  110,  112,  114,  121,  122, 
123,  127,  128,  129,  131,  132, 
133,  134,  135,  136,  137,  140, 
169 

horses  of,  98,  106 
North  Sea,  59,  139 
Northern  hemisphere,  129,  131 
NUTTALL,  73,  74,  76,  77 

Oaks,  American,  34 

Oceanic  islands,  141,  144 

Oklahoma,  130 

Old  World,  111,  114,  123,  131,  132, 

133,  134,  138,  140 
Oligocene  epoch,  87,  102,  103,  105, 

110,  112,  113,  115,  117 
Ontogeny,  57,  62 
Opossum,  127,  133,  134 
ORBIGNT,  A.  D',  11 
Order,  32,  125 
Ordovician  period,  87 
Oriental  region,  128,  129 
"Origin  of  Species,"  1,  17,  65,  83, 

169 

OSBORN,  111 


180 


INDEX 


Ostrich,  blood  of,  79 

wing  of,  50 
Otters,  32,  134 
Ox,  blood  of,  78 

fore  leg  of,  44,  45,  46 

Pacific,  islands  of  South,  142 
Palaeontology,  55,  82 
Palaeotheres,  107 
Palaeozoic  era,  87 

life,  91 

Paleocene  epoch,  87,  104 
Palingenetic,  59,  60 
Palms,  88,  120 

cocoa-nut,  143 
Pampas,  15 
Panama,  Isthmus  of,  131,  134,  138 

marine  fishes  of,  138 
Parasites,  72 
Patagonia,  135 
Peas,  158 
Peccaries,  134 
Penguins,  50 
Pennsylvania,  84,  109 
Perissodactyla,  112 
Permian  period,  87,  98 
"Philosophic  Zoblogique,"  8 
Phylogeny,  57 
Phylum,  32 
PICTET,  153 
Pigeons,  36 
Pigs,  144 
Plants,  Carboniferous,  95 

Cenozoic,  88 

Cretaceous,  93 

evolution  of,  166 

modification  of,  150 

of  White  Mts.,  130 

origin  of  cultivated,  28 


Pleistocene  epoch,  87,  99,  106,  109, 
116,  122,  129,  130,  131,  182, 
133,  138,  140 

Pliocene  epoch,  87,  101,  106,  107, 
108,  109,  113,  129 

Pliohippus,  108 

Poebrotherium,  115,  117 

Pointer,  36 

Porcupine,  Canada,  133 

Porto  Rico,  143 

Porto  Santo,  37,  39 

Potato-beetle,  155,  164 

Pouter,  36 

Pre-Cambrian  eras,  87 

Prefonnation,  62 

Procamdus,  115,  117 

Protohippus,  102,  105 

Protylopus,  115,  117 

Province,  zoological,  128 

Pterodactyls,  50 

Quadrupeds,  32,  89 
Quaternary  period,  87 

Rabbit,  European  wild,  37 

Falkland  Id.,  39 

Jamaica,  39 

Porto  Santo,  37 
Rabbits,  121,  134,  144 
Raccoons,  32,  134,  140 
Races,  geographical,  34 
Radius,  43,  44,  45,  100,  104, 114 
Rails,  146 
Ranunculus,  151 
Rats,  134,  144 
Raven,  49 
RAT,  7 

Realm,  zoological,  136 
Recapitulation  theory,  57,  61,  64, 

73 


INDEX 


181 


Recent  epoch,  87,  99,  101 

Saturnia  bolli,  40 

Recessive  characters,  158 

luna,  39,  40 

v  Record,  geological,  95 

Scandinavia,  181,  132 

palaeontological,  93 

SCHLOSSER,  171 

Region,      zoological,      1526,      197, 

SCHMANKEWTTSCH,  152 

136 

SCHHODEH,  153 

Reindeer,  132 

Scorpions,  65,  79,  92 

Reptiles,  33,  144 

SCOTT,  28 

age  of,  90 

Sea-lilies,  Cenozoic,  89 

Cenozoic,  89,  90 

Palaeozoic,  92 

Mesozoic,  90 

Sea-lions,  32 

Permian,  98 

Seals,  32 

Rhinoceros,  African,  111 

Segregation,  159 

Indian,  111,  112 

SELENKA,  143 

Siberian,  140 

Sera,  anti-carnivore,  78 

Sumatran,  111 

Serum,  74 

Rhinoceroses,  120 

anti-fowl,  75 

aquatic,  110,  111 

anti-horse,  75 

cursorial,  110 

anti-human,  75,  77 

evolution  of,  110 

anti-lizard,  79 

paired-horn,  113 

anti-llama,  78 

true,  110,  111 

anti-pig,  75,  78 

Rodentia,  see  Rodents. 

anti-turtle,  78 

Rodents,  103,  133 

Sexual  selection,  19 

rudimentary  teeth  in,  70 

Seychelles,  142 

porcupine-like,  134 

Shark,  embryo  of,  63 

Rudimentary  organs,  56,  66 

Sharks,  Cenozoic,  89 

Rudiments,  44 

Mesozoic,  90 

Ruminants,  blood  of,  78 

Palaeozoic,  91 

ontogeny  of,  69 

Sheep,  36,  132 

Russia,  170 

blood  of,  78 

embryo  of,  70 

Sabre-tooth  tigers,  123,  135 

Shells,  fresh-water,  144 

Sacculina,  72 

Sicily,  124 

Sago  palms,  88 

Silurian  period,  87 

Sahara,  128,  139 

Simiidse,  77 

St.  Helena,  birds  and  reptiles  of, 

Skunks,  134 

146 

Sloths,  134 

Salamander,  144,  156 

Snail,  garden,  161 

182 


INDEX 


Snails.  144 

Snakes,  blood  of,  78,  79 

Cenozoic,  89 

limbs  of,  67 
Solenhofen,  96 
Sonoran  region,  128,  131,  132,  133, 

134,  136,  138 

South  America,  15,  110,  114,  120, 
121,  123,  127,  128,  129,  130, 
131,  133,  134,  135,  136,  137, 
140,  145,  169,  170 

birds  of,  134 

horses  of,  106 

mammals  of,  130,  133 

Tertiary  mammals,  131,  169 
Spain,  123,  139 
Spaniel,  36 
Special  creation,  7,  29,  30,  35,  66, 

67,  86,  88,  120,  129,  137.  148 
Specific  centres,  125 
SPENCER,  18 
Sphenophyllales,  95 
Spiders,  65,  79,  144 

Palaeozoic,  92 
Splint-bones,  45 
Sports,  23 
Squirrels,  134 

rudimentary  teeth  of,  71 
Stag,  red.  132 
STANDFUSS,  153 
STEINMANN,  3,  98 
Straits  of  Magellan,  125 
STRANGEWAYS,  76 
Struggle  for  existence,  18,  24 
Styracosaurus,  119 
Sub-order,  32 
Subregion,  128 
Subspecies,  34 
Sumatra,  142,  143 


Survival  of  the  fittest,  18 
Switzerland,  39,  154 

Tadpole,  59,  156 
Tapirs,  123,  134 

distribution  of.  129.  130 

evolution,  109 
Tasmania,  127 

Tasmanian  Wolf,  blood  of,  78 
Teleosts,  89 
Terrier,  36 
Tertiary  period,  87,  98,  106,  127. 

129,  131 
Texas,  39,  154 
Thylacine,  blood  of,  78 
Tigers,  120 
Toad,  larvae  of,  156 
Toads,  144 
Torosaurus,  119 
TOWER,  155,  164,  166 
Transitional  zone,  132 
Triassic  period,  84,  87.  97 
Triceratops,  119 
Trilobites,  92 
Trochophore,  68 
TROXELL,  108 
TSCHERMAK,  24 
Tse-ste  flies,  126 
Tumbler,  36 
Tunicata,  65 
Tunis,  124 
Turtles,  blood  of.  78.  79 

Cenozoic,  89 
Type  of  structure,  42,  54,  55 

Ulna,  43,  45,  100,  103,  114.  116 
Uniformitarianism,  12 
Unit  characters,  158,  165 
United  States,  132,  135 


INDEX 


183 


Uranus,  171 
Utah,  171 

Variability.  17 

Variations,  23 

Varieties,  34 

Vegetation,  Carboniferous,  95 

Cenozoic,  88 

Mesozoic,  90 

Palaeozoic,  91 
Vertebrata,  32 
Vertebrates,  ontogeny  of,  63 

Palaeozoic,  91 
Virginia,  84 

VRIES,  H.  DE,  23,  24,  26 
VuLpes,  32 

WAAGEN,  22,  23 
WAGNER,  21,  40,  41 
WALCOTT,  85 
WALLACE,  13.  141 
Walrus,  32,  130 
Wapiti,  132 
Wasps,  89 
Water-fleas,  153 


Weasels,  32,  134 
Weeds,  European,  121 
WEISMANN,  20,  21,  22,  153 
West  Indies,  127,  128 

mammals  of,  143 
Western  hemisphere,  127,  136 
Whale,  33 

flipper  of,  42,  46,  47 

Greenland  Right,  47,  67 
Whales,  blood  of,  78 

development  of,  69 

limbs  of,  67 

toothed,  67 
White  Mts.,  130 
WILSON,  3,  26,  62 
Wolf,  32 
Wolverenes,  131 
Wolves,  32,  123,  134,  140 
Wonder  of  Peru,  159 
Worms,  embryo  of,  68 

ontogeny  of,  59,  65 
Wrist-joint,  43 

ZARCO,  37 
Zebras,  99 


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